Pls ask the mother. Be kind in ur words and ask her as to what she feels would be a good gift for him. Ask her to give at least 2/3 ideas so u can get at least 1. Or search online as to whats a good gift. Pls remember that autism makes each child differently unique. What worked with one autistic child may not work for another. Another thing is autistic children are highly intelligent. They r not of normal IQs. Very hi IQs. So it's better to do ur homework...
I think the best thing to do here would be to speak to the parents first and then pick a gift