That's a tough one. I don't know that you can just tell someone, "quit stressing" and have that get anywhere. And, it could be working the other way around, his physical illnesses could be causing depression and anxiety (lots of physical illnesses have that side effect) so I would see if I could convince him to get to a regular doctor for blood tests and physical and see if the doctor can help him worry less if that's the problem or can find a physical problem that he can treat if it's the other way around. But other than listening to and supporting him as best you can, I don't know that there's much that family members can do. Doesn't sound like, if it hasn't worked the rest of his life, that just talking to him will work?