There is a form of bipolar that is rapid cycling - usually that means several times a year, but there are people who swing back and forth several times a month, week or even a day...of course, I have no idea if this is what's happening with him and him seeing a doctor and being completely honest about his symptoms (including the anger and seemingly rapid mood shifts) would probably go a long way in getting him the proper kind of treatment. Not sure how steroids interact with bipolar without doing a little research, but I can imagine how they could throw one off.
I also like The Bipolar Handbook that nandm suggested - it's a great resource. Hope you're able to find some peace for YOU and take care of yourself! Maybe you could meet with a therapist on your own to discuss how to set boundaries and stick to them and keep yourself healthy and happy through all of this. It's very important to care for and nurture yourself.