Does the bully pick on your son alone, or does he have other targets?
If he has other targets, perhaps your son's best move is to stand up for one of the other kids being bullied. (Verbally. No violence. Just a "Hey, stop that!" And then walk with the other bullied kid out of the situation. Two people can walk away a lot more easily than one can.)
This demonstrates strength to the bully, earns the respect and gratitude of the kid being bullied, and starts to form a band to combat this kid's nastiness. Then the two kids stand up for the next kid, and so on.
Honestly, it's what I did as a kid, and it's the quickest way I know to stop bullying.
