Rupert Isaacson and I do not get off to a good start. I am grumpy because I've spent ages looking for him in his hotel and getting reception to call his room, finally running him to earth in a dark alcove in the bar. He is wearing jodhpurs and butch leather chaps, which in a slick hotel in the centre of London looks a tad pretentious - even if he has been riding that morning. I suggest we find somewhere with a bit more light so we go to the upstairs dining room and start the interview, which sometimes sounds more like a lecture. "What you need to understand is this ..."; "What your readers have to realise is ...”

Isaacson, it seems, is an expert on many things: on psychology, on horses, on dressage - he trained in Portugal - on autism: "I am an autism parent, I've tried most things," and on healing rites - "miracles happen all the time". I try to curb my irritation: after all, people who get things done against the odds - and Isaacson has achieved a great deal - do often seem didactic. Bob Geldof didn't shame the G8 into stumping up for famine by being charming, just as Isaacson didn't get land rights restored to the Kalahari Bushmen by going through the proper diplomatic channels (more of which later). And modesty didn't land him a $US1 million advance for a book about taking his son across the Mongolian desert to be healed by shamans - and that was before they'd even left home. The Horse Boy duly became a best seller, enabling Isaacson to set up a riding centre at his home in Texas providing therapy to children and young adults on the autistic spectrum. Local parents are not charged for the service; instead, he raises the $US200,000 annual running costs in a number of ways: applying for grants, giving dressage lessons, riding his horse into bars to ask for money, and training others in the Horse Boy Method, now replicated in centres around the United States and in the UK. "I didn't invent it," he says. "My son did.”

As a small child, Rowan Isaacson exhibited classic symptoms of autism: he didn't speak, he stimmed (self-stimulation with rocking or flapping of hands) and threw tantrums a dozen times a day. He was also doubly incontinent and rarely slept for more than a few hours at a time. Once he started walking, he could work off his distress and frustration by running into the bush around the family home. So during the day, while his exhausted wife, Kristin, went off to her job as a professor of psychology, Isaacson would shadow his son on his erratic path, sweating through the heat of a Texan summer. "Rowan needed to move, move, move for hours at a time," he writes, "me tagging behind in lonely silence as he dashed ahead, chirruping to himself in his private language from which I was firmly excluded.”

One day, Rowan switched from his usual route, barging through the undergrowth and into a field of horses. By the time his father caught up, Rowan was lying on the ground among the hooves, giggling. As Isaacson watched, heart in mouth, the alpha mare of the group shooed away the others and lowered her head towards the child, whiffling and licking her lips in a gesture of acceptance. From then on, Rowan wanted to visit his new friend, Betsy, every day. Soon Isaacson was putting him up on her back where he lay still and dreamy while she grazed; her owner lent them a saddle so father and son could ride together. One day, they watched a bird lift from a pond: "Heron," Rowan said. Isaacson was astonished: only a week earlier, speech therapists had said there was nothing more they could do, his son was unreachable. "Now here he was, bloody talking," he writes.

And there was something more: the faster they cantered on the horse, the more words Rowan produced. It was the start of everything. "If you put the horse into a collected canter," explains Isaacson, "it rocks the rider's hips in a way that produces oxytocin - the bliss hormone. Most people on the autism spectrum are ruled by the stress hormone cortisone: if you do something that replaces it with oxytocin" - he snaps his fingers - "what you get is communication. Also, when you ride behind a kid you are a voice in his ear, not the frontal challenge they usually have to defend themselves from, and holding the kid with deep pressure to your body is something a lot of them crave. Plus anything that causes you to find and re-find your balance - as happens on horseback - opens up the learning receptors, so the kid is in the perfect condition to receive and retain information.”

Isaacson is persuasive and, anyway, I've seen enough of the effect that horse riding has on people with disabilities to believe in its power, whatever the explanation. But, as he points out, not everyone has access to a horse, much less one trained for the purpose.

"We began to think about how we could reproduce the horse-rocking effect in other situations," he says, "on a swing, on a trampoline, on someone's shoulders." The result was Horse Boy Learning, developed as a non-equestrian alternative applicable in cities, parks and homes.

"We say, tailor the concept to the child in front of you," says Isaacson. "Always follow the child's lead."

At five years old, away from the soothing, pleasurable company of his beloved Betsy, Rowan was still severely autistic, still incontinent, still in the grip of neurological distress. His parents had explored orthodox therapies, dietary and environmental factors to little avail - then Isaacson had the glimmer of an idea.

In the late '90s, he had worked as a journalist in South Africa and helped a group of Bushmen in their quest to reclaim their traditional hunting grounds. A few years later, he played host to a group from the Kalahari, who were visiting the States to protest to the UN about being forced off their land. Rowan's autism had just been diagnosed and healers among the Bushmen offered to work with him. Isaacson had witnessed shamanic healing rites during his time in Africa and seen very sick people inexplicably recover: "I thought, why not? It was just prayer and song after all; the worst thing that could happen was nothing."

In the few days the Bushmen spent with Rowan, he began to lose some of his worst obsessive traits. Could there be a way to combine shamanic healing with horse riding? The resulting pilgrimage across the steppes of Mongolia to consult horse-loving shamans was epic, not least because it resulted in Rowan's first intentional bowel movement, squatting on a river bank and cleaning himself afterwards. Back home he was no longer incontinent, his tantrums had ceased, he started playing with neighbouring children and he rode Betsy solo. Buoyed by these results, Isaacson acquired more land, trained more horses and the New Trails Centre opened its doors to other children on the spectrum.

But Rowan was still autistic, still obsessive about routines, still not up to conversation and, within a year of the trip, was reverting to old behaviours. A Mongolian shaman had told Isaacson he would need to make three more journeys to complete his son's healing: it was time to start planning. "We were not looking for a cure," he says, "but further healing that would help Rowan be the best he could be.”

Over the next three years, the family would visit shamans in Namibia, Australia and Arizona. Isaacson's long-suffering wife Kristin had already been made to wash out her private parts with vodka in Mongolia; now more outlandish rituals would follow. In Namibia, as healers danced in firelight, pouring sweat, working themselves into a trance, one uttered a piercing shriek, dropped to the ground in a dead faint and took several hours to be revived.

In the Daintree Forest in far north Queensland, a Yalanji healer called Harold waved smoke from burning bark over Rowan and plucked at the air above his head, flicking drops of bloody mucus into a cup. After three of these sessions, Harold proffered tea and jam damper and told the Isaacsons: "The young feller'll be doing better now, you'll see."

In Arizona, a Navajo healer called Blue Horse placed a bleached bone against Rowan's neck, sucked the end and spat out a stream of black muck. The Isaacson parents were then required to sit in a humpy filled with burning hot stones to pray and sweat for several hours. Was it worth the effort? In The Long Ride Home, Isaacson's account of these extraordinary quests, published this week, he is unequivocal: everything has improved for Rowan, and therefore for his parents - language, cognition, behaviour, sleeping, stress levels.

What would have happened without those dramatic encounters with shamans? "You have to be comfortable saying you don't know," says Isaacson. "Perhaps it would have been something else." I suspect love, patience and inspiration have a great deal to answer for. Early on, when Rowan had almost no expressive language, his father always spoke to him as if he had, naming trees and birds, explaining atmosphere, why the sky was blue. Orthodox schooling had been tried and discarded: instead Rowan learns on the hoof, sometimes literally: basic maths comes by counting Betsy's footfalls, fractions from doing half and quarter turns with her in the arena; he recites his times tables bouncing on the trampoline.

If I met Rowan and introduced myself, what would he say? Isaacson smiles: "He'd say hello, then he might ask what year you were born and what car do you drive. But then he'd catch himself and do the 'social script'. After that he might engage you in unscripted conversation. He is highly functioning within the autistic spectrum, but it's taken a long time.”

Things have not always run smoothly: Isaacson had a spell of drinking too much and he and Kristin have recently split up after 20 years of marriage. "We'd been picking at each other," says Isaacson. "Special needs kids expose cracks in a relationship and 90 per cent of marriages break up under the strain."

He and Kristin remain friends with separate houses on the farm. "When I'm not travelling I'm always there to see Rowan to bed and have breakfast with him in the morning," he says. "I think we'll stay together as a family unit."

Does he worry about the future for his son? "No more than any parent," he says. "My gut tells me he will probably live within the family setting, in his own house on the compound and be part of the family business, as he is already." He frowns: "My only worry is that one day I won't be there for him."

They would have liked a sibling for Rowan: "But by the time we were out of the trenches in the early years, Kristin was 40 and it was a bit late."

Several of the staff at the centre have worked for him for years - "We're a bunch of horse hippies" - and he is fond of the saying that it takes a village to raise a child. So surely Rowan will be safe within the tribe won't he? "Absolutely."

There are some wonderfully moving and sometimes hilarious moments in Isaacson's book. Rowan never shares food, but on one occasion he offered his father his box of chicken nuggets. Inside was a lifelike latex toy squid. As Isaacson recoiled in alarm, Rowan collapsed in delighted giggles: "It was a squid! Not chicken! A squid! Daddy thought it was chicken." Isaacson joined in the joke, but psychologist Kristin was thoughtful: a practical joke involves the ability to see something from another person's perspective, she explained, an important developmental milestone that had once seemed way beyond their son's reach.

These days Rowan has his own small zoo of animals, which he introduces to visiting children. One day he showed a small, silent autistic boy how to make a duck quack. "You pick him up like this and press your finger here," he said. "Quack," went the duck. "Quack," said the little boy delightedly. And then: "Ducks go quack." His mother burst into tears: they were the most words he had ever spoken.

During the family's overseas adventures, Rowan became passionate about wild animals and coined the word "endangerous" for those beasts both dangerous and endangered. A TV program is now in the making, conceived and scripted by Rowan, and the family is off to Brazil later this year to film jaguars.

"Autism is the best thing that ever happened to me," says Isaacson. "What a privilege to live this adventure, to be the dad of this boy."