You're a smart kid, just the fact that you are asking for guidance leads me to believe you already know what the right thing to do is and you just need confirmation of that. As was said before if he hurts someone you will be burdened with that guilt for the remainder of your life and that would be a horrible thing. If he is only joking it is definitely a bad joke and he should learn the lesson that to joke about something so foolish is not without consequences. Take heart that you are doing the absolute right thing by informing an adult who can handle the matter.