My son put his hands on another child's neck and?

Explain to him how he can not do that (very calmly). Give him a time out or whatever punishment you usually use (so that he knows it was wrong). If he see's a counselor/therapist/social coach etc (which most children with any form of Autism sees) than let them know and see if they can help bring some of it into their studies on that its not a thing we do.