I realize he's a boy, so these might not fit, but American Girl Dolls has a really great series on friendships, bullying and peer pressure. We read and talked about a lot of these issues when my daughter was around 6 or 7 and her BFF was suddenly very controlling and mean to her. We talked about the kind of person my dd wanted to be and talked about how her actions and interactions with people were so important. I made a big effort to talk about how friends behave (and don't behave), and how ethical and moral people behave. I would use TV moments or other observations to ask, "do you think that person is being a good friend?" "What do you think that other person thinks about what he just said?" "What would you do in that situation?" It was an ongoing dialogue then that will probably always continue.
I think parents have to be aware and have intentional conversations so that kids will know when they're being bullied (my dd didn't always recognize when her friend was bullying her) or pressured. But I think the conversations should also be about what to do when they see other kids who are the focus of the behavior, and give them ideas on how they can handle the situation.
