It's not a safety issue. It's a discipline issue. He is perfectly safe.
Your son was attacked and the kids who did it received little or no punishment and yet you are saying it is not a safety issue? I suppose then you must have received a signed and notarized statement from the two angelic little attackers promising to never do it again since they are certain to have changed their ways without receiving so much as detention?
You already said they turned around and did it to another student yet you are saying it is not a safety issue?