If you are looking for information on what it is like to live with someone with depression and/or bipolar, Anne Sheffield's book, how to survive when they are depressed covers both illnesses from the p.o.v of the spouse/family of the ill person. It aims to help you set boundaries and stay sane. The author is both a sufferer of depression and also had a depressed parent, and has done a lot of research into living with depression fallout.
She mentions often in her book, but I am probably misquoting badly so this is just the jist that I understood, that denial, lying and distorting the truth can be an aspect of the illness. That might explain why he has hidden it, or fear of abandonment. Still...
She has a good message board too, if you google 'depression fallout'. Again covers both illnesses. You might get more experienced insight there as to what living with a bipolar sufferer can be like, and whether this matches what you have experienced.
If he does indeed have bipolar, my (very limited) understanding is that it is not fixable, but controllable through drugs and therapy, and that importantly those things are needed. It would be worth finding out whether he was professionally diagnosed and why he has not followed through with treatment. Possibly, consider encouraging him to return to a GP and discuss it with them together. Particularly, as the behaviours what you are attributing to this as are you say draining on you. I also understand that it is not uncommon that people suffering from bipolar that is not treated to also then become depressed, now that does not sound like double the fun to me!