if I were you I would just start it on a loose conversation with the parents, or mother. Just ask her if he does it at home with her because he has been doing it with you. I dont think it would be that big of a deal to address with the mom, and that way hopefully she will be perceptive enough to respond by saying something like, "oh, i guess I never thought about him doing it to others, maybe its something we could work on at home as well". just let her know you are a little concerned over his social interaction with the other kids. gently let her know some of the kids tend to avoid him because he invades their space a little too much. I do know through experience, it sure makes things easier on everyone if parents work on little issues at home as well as you working on things at your house.