S.P.
Hi C.,
I realize being austic - it may take different ways to stop the hitting, do you try "time-out" and tell him why? Does certain things set him off? Overstimulated/tired? Do you take any food or something special away/remind him why to let him know what you want from him? Since I don't know the extent of his austism, if possible try to trace what sets this behavior off. Is it something he wants/scared?
I know it can be very frustrating. Hang in there. Best Wishes!