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.
I just wanted to second this recommendation - the AG books are great! And yes, I have a son in addition to daughters
ITA with everything syoblrig said, and will add that I would not worry about what's happening now be a predictor of troubles as a teen (and likewise I wouldn't predict that the 6 year olds who are not caving in to peer pressure now will somehow magically escape peer pressure when they are teens!). One of my dds went through this in early elementary, and one thing that made it tough was the school situation she was in - she switched schools in 5th grade, and it wasn't until after the switch that I realized how much a school culture can really sway peer attitudes and create an atmosphere of respect or conversely allow an atmosphere of bullying and acceptance of poor behaviors. Not that that will help you necessarily, but you might watch and see if this seems to be something that is happening with just a few kids or if it's widespread among peers at school.
Best wishes,
polarbear
