The best way to treat bullies is with a Jack Dempsey attack. At the sound of the bell, come out swinging before your opponent knows what is happening. Sure, the bully will try to punch back but the centrifugal force will be on your son's side and the bully should be pushed back, crashing into lockers, toilet stall doors, or wherever the bullying was taking place. And don't let your son feel guilty about being expelled. When he returns after a few days, the girls will think he's a hero. At least this worked for me, back in the 1960's.
I don't condone fighting, but Rikki Gins is right. I learned this when my silvertip black lab and my temporary rescue American bulldog became unsequestered one time. Oops. I silently said goodbye to my lab, prepared myself for his death. But he was experienced, and the bulldog was not. My lab won the fight, fought fast and hard, probably with an element of surprise. The bulldog stopped after he got bit a couple times. I was really amazed, because the bulldog seemed by far the stronger, more dangerous breed.
But my dog did pull a disappearing act when two huge rottweillers were running at large on a beach we were at. I cut through yards, too, but he got home way before I did. Size, age, backup, experience, surprise, and home advantage ALL matter. And that's not even considering weapons. Creepy.