Why do you think he's bipolar in particular? He sounds depressed. And his cynicism is probably a product of that.

If he has "high" elated moods then it would be helpful for him to see a psyhchiatrist. Otherwise have a go at dragging him to a doctor forcibly! (or use emotional blackmail, I don't know, make him go...) and go with him (even if he protests) so that the doctor gets the "full picture" from an objective as well as subjective point of view.

I know this sounds hard to do, and maybe a little cruel, but this is what my Mam has done for me when I've been really bad and it's always meant a change for the better in one way or another.

I'm afraid this is the only advice I can think to give.

I hope it's helpful.

Good luck.