At kindy/school, they teach kids to put up a hand and say "Stop it, I don't like it." If that doesn't work they are encouraged to seek help from a grown up or a peer mediator.

It might help to explain to him what his choices are. He could stay and put up with it, or he could move away, or he could tell a grown up, or he could tell them he is going to tell a grown up (the threat can work just as well as the actual telling).