I'm not medically qualified, but it does sound like he's either bipolar or BPD (borderline personality disorder). Being close to someone with either of these conditions will certainly put you through the wringer, I can testify to that. I've had two friends who were diagnosed bipolar, and it eventually wore me slick and I had to detach. I don't know what to say except do it as long as you can, and when you can't anymore, then stop. They're like an alcoholic in that until they hit bottom and decide on their own they need help, there's really not much you can do. Often they do something really outrageous and get arrested and that's how they get into treatment (if they're lucky enough to live in a society that knows the difference between mental illness and criminality).
I wish I could be more optomistic. Maybe somebody with more experience and qualifications can give you better suggestions about what you can do. All I learned is that you aren't really helping them if you play along with their moods and bail them out of all their difficulties. They can always take it farther, and eventually that's what it will come to. If you fear for your physical safety, and you're as emotionally drained as you sound, it may be time to distance yourself from him with love and let him go his own way. Here's a link that may help. A couple of web searches will yield tons more, I'm sure.
www.nimh.nih.gov/publicat/bpd.cfm
Here's another one that looks pretty good:
http://www.mhsanctuary.com/borderlin...st/askther.htm