mmzz - I'm not a DM of a DS but I would say that your DS is now of an age where you need to let him deal with any friendship issues ...its hard being a parent when you think there is underlying bullying but this maybe general banter and it seems that the group seem to keep the aggressive member in check (from what you posted).
DD has been bullied extensively over the years (and I worried myself to sleep on lots of occasions). I think she is mature enough to handle things now and in fact she plays the peacemaker most of the time now....because she can see both sides of conflict and has a pretty balanced view on life (except when she has a period).
Sometimes when kids are bullied you see this reflected in other ways like poor results (e,g, failing exams) but your DS seems to have not let any of this affect him so I would not highlight an issue that doesnt seem to bother him unless he shows any other signs that he is not happy.
Dd hasnt found her tribe either ...but she's learning valuable life skills dealing with an interesting friendship group where you have Alpha females, a few sheep and anything in-between.