First of all, I think it is admirable that you want to do something for him. That is very caring.
Like the others say, I don’t think there is anything you can do personally. But, there may be something you can do indirectly. Perhaps you can work to alert his support system (whatever it is) to the things they can do.
To some degree, you’ve already done this, when you tried to talk to the girlfriend. Even though he threatened you, you could still try to find a way to talk to her where he doesn’t know. This could be dangerous if he does find out. I would really want to know more about her—how old she is; how mature she is, in order to assess whether I would want to attempt this step.
However, perhaps there are others in his support system that you can talk to. Parents? Siblings? Other relatives? You must know of them, even if you haven’t met them. Perhaps they would want to try to care for him.
Also, we don’t know what his mental problem is. If it is Bipolar, then he will move into depression at some point, and that would be a better time to try to help then during mania. That could be the most important time, too, as that’s when he may get closest to suicide.
If he’s got some other mental illness, I can’t really help, because I haven’t experienced them. I have often wondered about Schizophrenia, because I think I might be able to relate. But that’s a whole ‘nother story. Good luck. And thanks for caring.