One of the characteristics of autism is a lack of empathy, which means that he has no idea how his own behaviour makes other people feel. He literally cannot imagine how you feel. However, his behaviour was not acceptable, and by 12 years old I would have expected someone to have taught him that.
Having worked with people with autism, I personally feel that the blame here lies with his caregivers, because he should have been taught about appropriate sexual behaviour. I feel that if someone doesn't have the necessary knowledge to behave appropriately then it's up to their carers (whoever they are, parents or support workers or whoever) to monitor them and help them learn how to behave.
Mim