There is no way around it: When your child gets to the preteen age, he's going to be dealing with a number of social pressures. These come from both the inside and out, as children struggle to define themselves and control how they are viewed by others.
Have a conversation with your child early on in his life regarding peer pressure. To an adult, it might sound a bit clichéd to have a talk about thinking for yourself or not letting others get to you, but preteens will often seek approval from peers, even ones they're not particularly fond of. This can lead to dangerous situations regarding sex, violence, drugs or alcohol, so it's important to talk to your child often and early.
Ensuring your child's self-esteem stays and that his identity is solid can help him deal with peer pressures, but don't lay it on too thick. After all, parent's compliments can only go so far with a preteen.