I know a small fraction about autism compared to people like @apm , but I guess Wiltshire is classified as autistic savant?
Yes, he has autism and would be considered a savant for his extraordinary talent with art. As a child, his art teachers forced him to learn to talk by taking away his art supplies so that he would have to.
At the age of five, Stephen was sent to Queensmill School in London, The instructors encouraged him to speak by temporarily taking away his art supplies so that he would be forced to ask for them. Stephen responded by making sounds and eventually uttered his first word - "paper." He learned to speak fully at the age of nine.