I think that middle school, particularly 6th and 7th grades, are the most difficult/stressful for kids to navigate. There will always be kids who try to dominate and exclude. If your DS is willing to discuss the particular details with you, then you may be better able to determine whether he is a actual target of bullying. In general, if he can focus on like-minded peers and perhaps get a feel for how they feel about these boys, it can be a relief for him.