I think the key here is for Doc to minimize the potential damage that might eventually head his way. Yes, depraved indifference might be very hard to prove, and yes, there isn't much you can do if this guy's family tries to create trouble for you. But somewhere in the middle are appropriate actions that you can take now that might ultimately minimize the hassle and tumult in your life if this guy actually does go through with it.
If you live in a house on a coast, and you hear about a hurricane headed your way. Nobody knows if it's going to swing north of you, or south of you, or possibly blow right through you. Only a fool looks at this situation and thinks, "Well, I've got a 2 in 3 chance that this storm won't hit me. Those are some good odds, so I'll just kick it and it'll all be fine." Now, you might not pack up everything you own and head for the hills, but there are reasonable steps you can take that help mitigate against disaster.
Really, just talk to someone - cops, docs, shrinks, his family, anyone that could conceivably do something besides just advise you. I'd probably do it just because of FATTY's point. If he offs himself, that might be one thing. But if he decides to take a few innocents with him, I don't really need that on my conscience.