My son, Rupert Isaacson, is a writer, horseman and campaigner for indigenous land rights, now living in Texas. After Rowan, his longed for son, was diagnosed as autistic at the age of two and a half, a determination to turn disaster into an adventure grew out of his despair.

One day his toddler son ducked under a fence and lay down among the hooves of a band of horses. Rupert's fear turned to wonder as Betsy, the alpha mare, gently whiffled at the child and stood quietly while Rowan gazed upwards and Rupert carefully retrieved his son. Seeing Rowan so relaxed, Rupert began riding Betsy holding Rowan in front of him. Soon an idea took shape. Rupert decided to combine his two greatest passions, horses and the indigenous healing practises he had witnessed, into a wild enterprise - to ride through Mongolia with his deeply autistic little boy in search of shamans (healers) to help him. His book, The Horse Boy, tells the story.

Ghoste, the shaman in Siberian Mongolia, instructed Rupert to make three more journeys to other shamans to make the healing complete before Rowan was nine. Rupert's second book, just published, is The Long Ride Home, and it tells the story of these journeys and the course their lives have taken in the years that followed. My son invited me to come on the first of these journeys, to Namibia. I am South African born, and had accompanied him on a trip to the Khomani bushmen in South Africa in the euphoric period at the start of their land claim. Rupert took Rowan on two other journeys to the Rainforest in Queensland, Australia and the Navajo Reservation in America - both with extraordinary results.

Adventures come naturally to Rowan. In Namibia, we slept in two tents on the Land Cruiser roof, mother and father in one, grandmother and grandson rolled together in the other. The Bushmen in the healing village are quiet and calm people as we experienced them, the healers dignified and kind. Rowan liked them - particularly the healer's wife who was a great comedienne. He was peacefully at ease during the night time healing dances. On the second night, after several laying on of hands, he got up, walked out of the circle to a woman with a baby and ran his hands over the little velvet skull. The healers stopped. One said, "He wants to be with the children now", and the ceremony stopped without fuss.

As his grandmother, my relationship with Rowan is deep, in spite of living either side of the pond. Even when most deeply shut away, he was responsive to close hugs and rhymes and songs. Our relationship was warmly physical. He could relax in my arms even when distressed.

Having been closely involved with many different infants, we, his grandparents, noticed Rowan's developmental differences from his first year. We asked careful questions such as, "was he saying mama?", but dared not suggest there was cause for concern before it became apparent to his parents for fear of alienating them. However, unexpectedly, the autism diagnosis did put a strain on my relationship with my son. Help was and was not wanted. There was no debate. Rupert and Kristin tackled it alone. But our family has always stuck together and grandmothers play a vital role in all children's lives.

In Rowan's infant years, I went to Texas as backup for Kristin when Rupert was away working. When they came to London, they left Rowan with me to take a break for a few days. Then Rowan would sleep in my bed - or wake. I don't mind disturbed nights. I would help him sleep by rolling, bouncing, rocking and singing and looking at the stars. I have tried to move Rowan towards social normality more than my son liked, such as not eating his meals on the floor but at table, made endurable by reading aloud The Lion King for about four years on the trot.

Rowan holds court in London every summer, with visits to family members in Wales. He has developed particular and distinct relationships with his aunt Hannah and his older cousins Zoe and Lewis, all with their particular jokes and games. He has a great capacity to inspire dedication and the many volunteers who have worked at Rupert's therapeutic centre in Texas have grown to love him and come back to visit him.

When he was little, it could be heart wrenching when he was overcome with longing for something he felt was lacking. For a time he wanted a sister very badly. As his experience of supply was supermarkets, he begged his mother, in tones of real anguish, "Where can I buy a sister? When can I be a boy who has a little girl sister?". But this was longing from a distance. Playgrounds and actual children bewildered and alarmed him. He still prefers adults, with whom he is entirely comfortable and confident.

This summer he has said firmly that he wants to stay in London, have people visit him here and not travel about. It is a deep satisfaction to see him take control of his life in this way. My role is to make him his books. His 2013 book weighed nearly six pounds - too expensive to post - so he had to wait months for me to bring it on my April visit to Texas. I will break events into more manageable volumes in future.

From the time Rowan was about three, I have written and illustrated books about him and his doings, to help him understand who he was. Later, I noticed that he looked intently at photograph albums so I began a habit of photographing our times together, and assembling the pictures in books, linked by narrative and rhymes and jokes. Some of these rhymes have become very important to him as reminders of happy states and he prompts "Oh how Rowan remembers...", and I have to recite the whole passage, to his deep pleasure. He pores endlessly over these photo books.

I am less important to Rowan now. He has his beloved entourage in the form of his parents, his mentor Jenny and Iliana and the many volunteers who help at New Trails, the centre his father has set up to help other autistic families. He has his own purpose in life and has set up - quoting his own words - "The New Trails Zoo and Wildlife Sanctuary, a forever zoo whose mission it is to rescue and protect animals from all seven continents". He has a very special relationship with his ducks at the moment. He holds them close to his heart, his face serene and beautiful. The ducks apparently feel the same way, laying their necks on his shoulder. Skin and feather, warm together.

He is a fine strong boy and will be thirteen at the end of the year. He is apprehensive about leaving the protected state of childhood, and with good reason. But he has his tribe at New Trails to support him. He is such a loving boy, he will need someone to love physically in due course, but sufficient unto the day...

Rupert Isaacson's book The Long Ride Home is out today and is published by Penguin Books.

We also have 10 copies of The Long Ride Home to give away to people who post on the thread.