I would be talking to the teacher again, so she can be sure she knows when to intervene and separate them, and so she can help integrate this other child with making new friends... not just 'suggest' but actively put him in different groups and such. I would talk to the mom, let him know that her son is showing possessive behavior and that you are bringing it to her attention so she knows. Ask her if he ever has playdates or talks about being friends with other children. If you are friends, surely you can think of a nice and tactful way to bring it up. Definitely talk to your son and let him now it's perfectly okay for him to make new friends. let him know that the next time this child physically pulls him away, he can immediately go to to the teacher... that would not be allowed at my son's school.