I'm sure it's very difiicult to watch your son go through such a hard time with his friends right now. At the age of 11, kids can be cruel when it comes to picking friends. You mentioned that your son said his friends are mean to him. What does he mean by that? What are his friends doing that he feels is mean? Are they calling him names, bullying him, or excluding him from activities? Did something happen recently, or has he felt like nobody has liked him for awhile? Get as many specifics as you can. While talking to him, reassure him that he isn't a bad or unlikable person, sometimes when people don't feel good about themselves they try to make other people feel bad to make up for it.
The best thing you can do for him is to give him continuous praise and reassurance. Remind him that a true friend will stick by you no matter what. If his friends have been treating him badly, maybe they're not the right kind of friends in the first place. Encourage him to get involved in activities where he can make new friends outside of school such as sports teams, or a faith based youth group. Making new friends will give him the confidence he is lacking right now.
All kids go through troubles with peers at some point in their adolescent years. The key to getting through it is to have people around him that care about him and are simply there when he needs them. Make sure he knows you care about him and will always be there to listen if he needs someone. If he doesn't feel comfortable with you, remind him that he can always talk to a school counselor, or call our teen hotline at 1-800-448-3000.
I recommend an excellent book that I've used to help a few of my students who were victimized by the poor behavior of other students, "Words Will NEVER Hurt Me" by Sally Northway Ogden. She has practical advice on role-playing scenarios you can use with your son so he can improve his skills in effectively handling the meanness. Practice with him and rehearse his responses to his former friends. The former "victims" in my class were amazed at how enemies turned into friends with just a few of these rehearsed responses.
What happens is that your child displays higher self-esteem and sensitivity to the poor self-esteem of others (which is why they're mean to him in the first place). This helps him to gain the respect of former enemies.