He sounds like a bully and if the supervisor won't listen to you go to some one else ( the priest?). Your instinct is telling you that they way the man is behaving is wrong, and it certainly sounds appalling
I couldn't stand by and watch such nasty abuse.
He isn't treating the child with any respect at all. Being autistic doesn't mean that you have to be treated in such a disgusting manner, it means that you have to have understanding, and ways of stopping behaviour that is unwanted. For the little boys sake get the man to stop.
If no one will listen then tell the parents, I'm sure they'll stop it!