My goodness, that's horrible. I would do things like that as a teenager too.

I'd hate to set him off but if his safety is at risk, you have to do what you think is right (that might mean forcing him into hospital or therapy or even installing a keystroke recorder on your computer to monitor him). Maybe he would be willing to join into some type of family therapy if he won't do it alone. The fact that he doesn't hide it from you must mean he trusts you. I don't know what to say.

Wishing your family the best.
-Jeff