The other child maybe being abused. Especially if he is the one that wants to play in that way. You should talk to your little boy and see why he keeps playing like that. If you suspect abuse you need to report it. They both need to be watched together, and until you figure out the whole story you need to watch your little boy closely with other children.
Physcological help maybe smart but I would talk to your child first. See if you can get the whole picture and find out what is causing him to do this.
Hi there. Well, really, after the first time, these kids must be supervised. Who is the instigator? Could that child have been molested by someone as child upon child molestation is a common cycle. I personally would not allow your child alone with other kids and seek professional help with a psychologist as this is an ongoing problem. good luck