Maybe if you send him more death threats they'll leave you alone.
"You know, minimum security prison is no picnic. I had a client in there once. He said the trick is: kick someone's ass the first day, or become somebody's bitch. Then everything will be alright."
But seriously, apologize, say that you were just trying to get a laugh out of the kid, and that you realize it was a stupid and inappropriate thing to do. That's all you can do.