But Durins B. has wisely counselled his son to break that chain, bob.
My two NT = this is one of those situations that calls out for formal complaints. If the school responds in the way that other posters have predicted, not only does that absolve them of blame (if it’s let go, it never really happened) but there’s also no guarantee that the boy will not continue to target your son.
Scare that child (through, and along with his family/formal complaints) and make him understand that he endangered your son’s life. Otherwise, bullies just grow worse over time.