Mainstream or Special needs School for my autistic 5 yr old?

personally i think you are doing the right thing as he will be attending a school that best suits his needs, if the school is one that has a good reputation then its likely he will receive one on one attention and be able to better his skills. It sounds as if he is a fast learner, however he may find that he is met by a lot of criticism in a main stream school mainly by other children which may cause him become distant and refuse to participate in many of his lessons. If he carries on to Excel the way he is doing now there may be a good chance he will be able to Attend a main stream school or college in the future there is no reason to rule this out however ensuring he has the best start in an environment that best suits his needs is a very good idea.