Autism affects your ability to communicate or understand so he chooses to be quiet so people don't make fun of him… There's a backstory and I can't change that without changing the fact that he's autistic.
I do know what autism is. My cousin is very autistic. I don't think you should change that he's autistic, or that's he's quiet so people don't make fun of him. I like both of those things about his character. There's a difference between being quiet so people don't make fun of you and having trust issues, though.