As a 3-year-old, British artist Stephen Wiltshire couldn't utter a word, but his incredibly accurate sketches of elephants, giraffes and tigers spoke volumes. It was 1977 and his mother, Geneva - she was raising Stephen and his sister, Annette, on her own after their father, Colvin, 35, died in a motorcycle accident - discovered that her boy's language was art.

That year, Wiltshire was diagnosed with autism. Two years later, teachers at his London special-needs school limited his access to paper and pens, "which forced him to ask," explains Annette, 37. "His enthusiasm for landmarks arose, so they taught him to write incorporating them in the alphabet, like B for Buckingham Palace. They used his passion as a power source to make Stephen come out of his world."

Back then, the mute child's world was limited. "There was huge ignorance about autism," says Annette, art director of the Stephen Wiltshire Gallery in London. "Mum was blamed because she was a single parent."

These days, Wiltshire's world is spectacular. His art is in high demand and he travels the globe drawing city skylines (including New York, Tokyo, Dubai and Jerusalem) and educating people about art and autism through demonstration. For his latest project, for Autism Spectrum Australia (Aspect), he travelled to Sydney for Autism Month to draw the harbour-side cityscape. "Sydney was just like the American-city style," Wiltshire told WHO at Sydney's Customs House on April 29, displaying the startling image he drew from memory after taking in the view from Sydney Tower for 40 minutes two days earlier. "It's not so difficult," says Wiltshire, 36, who memorises by using "my head and imagination and how it looks a bit like similar things."

So how does he achieve such a feat? "Like all autistics, Stephen has a very visual memory," says Dr Trevor Clark, Aspect's executive director of education and research. "Autism is a neurological disorder that results in a visual strength, but people with autism have trouble using executive functioning, which is done in the brain's frontal lobe and deals with high-level thought. Stephen displays a prodigious savant skill and savants have heightened visual processing and the ability to view the world in fine detail."

Early intervention, mentoring and nurturing their special skills are crucial for people with autism. Stephen's artistic talent was encouraged but his obsessive desire to draw was the driving force. "We didn't have a choice!" says Annette. "It was Stephen telling us what he wanted rather than the other way around. As a family, we've never shown any negativity."

As a result, the young man has flourished. He studied fine art at City & Guilds of London Art School, and in 2006 was awarded an MBE by Prince Charles for his contribution to the art world. But he's still enamoured with cityscapes. He says he loves "drawing London landmarks and street scenes, taxis and big American cars the shapes of boats, like Cadillacs and Chevys, Lincolns, Mercuries, Pontiacs, Buicks and Oldsmobiles."

Like many people with autism, routine also plays an important part in his life. "Stephen has the most impeccable sense of time," says Annette. "You can tell by the minute when he'll walk in the gallery. It can be awkward because he has to leave at four and often visitors want to talk to him but he doesn't stop to talk. I explain to them that routine is key."

Now, Wiltshire, who lives in London with Geneva, 63, has moderately impaired communication skills that have been developed with a lot of practice and 15 years of speech therapy. He's keen to find a girlfriend, enjoys music, dancing and crime shows such as CSI, but he doesn't grasp what sets him apart. "He has no understanding of autism as we've never talked about it, because in the 1970s we didn't know what it was," says Annette. "Then we thought, 'What does it matter? His wiring is slightly different. We're all different in our own way so why should we focus on that?' "

In person, Wiltshire exudes a pure sense of joy, and for many he is an inspiration. "People say, 'You've done so much for Stephen,' but it's Stephen who's done so much for us," says Annette, smiling at her brother. "He has no fear, is very positive about what he wants and doesn't let anyone come between him and his passion."