@freddie11 Sounds like standard bullying nonsense.
Was your son hurt?
Bullies are the absolute worst. This is an important lesson for your son - he cannot control what anyone else does, he can only control what he does.
Disciplinary action for the bully is up to the school and the bully's parents to worry about. For you, your primary concern is keeping your son safe and making sure this does not become a pattern.
For that I can only suggest that you tell him to stand up for himself.
I know we all tell our kids not to get into fights, but at 13 he is old enough to get into a scrap if he has to. It's a last resort of course, but bullies don't pick on the kids who hit back.
To that end, if your son doesn't already have the tools to protect himself, I strongly recommend you consider having him take up wrestling if the school offers it, boxing or Brazilian Jiu Jitsu. These are sports that will teach your son how to protect himself.
If your son can learn to keep his hands up and his chin down, bullies will move on. If he learns how to take a bully to the ground and keep him there, he'll be able to protect himself without getting in trouble.