Here's a second-hand suggestion:
Someone I know told her son who was being bullied that it was okay to fight back. So what did he do? The very next time he saw the bully, he marched right up and told the kid "My mom said that if you push me, I can push back!"
The bully freaked out a little, asked the mom if that was really true, and then left the kid alone!
I think you can file that example in the "sometimes you just have to get back in their face" pile with what 'Neato said.
This was also my personal experience.
Finding a friend to tell the bully to stop helps, too. Bullies look for lone individuals. Even two people are usually more than a bully wants to deal with. Have your son find other loners and they can band together to protect one another. Chances are incredibly good that no one will need to fight anyone if the bully realizes that none of the loners are actually alone.
