First, I think it's wonderful that your son came to you.
I did a program at my daughter's school when she was a little younger than your son. It's a simple principle and I was aided by the fact that 20/20 actually did a program on bullying about two weeks before I went into the school to do the class. I used that tape as part of the program.
I think what most children don't stop to think about is what others feel when they are teased or being bothered by a bully. Roll playing games are fabulous - you can find some on the internet by searching google. Since you are in control of the situation, choose some of the kids that don't outwardly look like they would be teased. Have them be the ones made fun of in your role play. Involve as many kids as you can so that the embarassment factor is great. In other words, there are 10 kids laughing at one kid.
After a few of those, address the emotions. Lead a discussion on how it feels to be singled out, to be teased, to be left behind, etc. Address what responsibility they feel for stepping in when they see a fellow student being harrassed.
To end, give each child one piece of paper for each child, besides themselves, in the class. For example, there are 15 kids, each child gets 14. You can do a little work ahead of time (this is what I did) by writting a child's name on a piece of paper and then copying it as many times as you need. I find the less writing the kids have to do the better!
Have each student write one positive remark about each classmate. Prompt them if needed with things like - "One thing I like about A is...", "I have always liked when A....", "I wish I could ....... like A", "A is really good at ..... and I admire that." They don't include their name on the page.
Now, this make take a little time, but these kids are certainly old enough to finish this project. I went a little farther and gathered all the papers and took them home. I did a page on each student and filled it with all the comments their classmates had made. Then I made a little book (just stapled) and called it "The Good In Me".
It was so successful that the principal asked me to come in and do the same for all the other upper grade classrooms. And, I saw the mother of one of the girls in easy child's class about three years later and she mentioned that book!
It's really a way for each child to understand that there is something about them that makes them special and that there are other students out there who think positively about them. It's a real self image builder.
I think, whether you do the positive thing project (which if I remember correctly is a story from an old gentleman who carried a piece of paper with him all his life and when he was about to die, he shared it. His CO in the army years and years before had his unit do this exercise to boost morale - he kept it with him every day) or not, the role play is really effective with your son's age group. It is also important to discuss their feelings.
As a Sunday School teacher of teens, I have found that getting the kids really comfortable physically helps in the sharing of emotion and intellect! Perhaps the discussion could take place sitting on the floor in a circle, sitting outside on the grass if you are in a warm clime, or sitting in the library where there is probably some soft carpet. Once one kid shares, it's easier for the rest of them to "spill it"!!!!
Good luck.
Sharon