His parents or guardian(s), and then maybe if not then, his significant other, or his best friend if it isn't you, or someone with deep emotional ties to coax him into thinking clearly. The police are a last resort.
If you are on your own, which I highly advise against, because people tend to believe they are Jesus of the psychological world in their heads, but they can never truly know exactly what to say when the time comes to say something, but to get into morality, get him to talk to his teachers about punishing the bullies, not to take matters into his own hands. Murder, or threats like that in general usually mean the kid has issues himself. He isn't thinking rationally. Get him to trust the teachers and/or his parents/guardians.
and then of course there is the possibility the knife gives him a false sense of security and he will not end up using it when the time comes for him to "fight back."
hope this helps.
