I see. Well, since your son didn't say anything I'm sure he'll be fine. This isn't a situation where any criminal charges will be laid, so my best recommendation is to ride it out for now, unless and until someone wants to reprimand your son directly or have anything added to his school file which might follow him.
There's no need to involve the police at this point. You don't need an occurrence report which contains your son's name, and the police won't do anything anyway until someone makes a complaint which could lead to criminal charges.
If it looks like your son will be getting a direct reprimand and you want to stop it, then get a lawyer to write a letter to the principal and the school board saying that if they put anything in his record about it then you're going to get litigious. They'll likely back down.
There's also a lesson for your son here, obviously. He'll be judged by the company he keeps, and he should have reprimanded his friend at the scene, then this might not have gone so far.
Anything else about this to discuss? If so then please reply. If not, then I'd appreciate a positive service rating please.