First let me say that this is a difficult situation, and you don't know for sure if bipolar is the cause of your boss's unusual and counterproductive behavior. Regardless of the cause, the way to respond will be similar and will vary a lot depending on other aspects of each unique case.
It sounds like you suspect your boss is either untreated or the treatment is not working, because the behaviot is affecting your boss's ability to perform his job to his normal standards.
You ask whether to try to get help or to just work someplace else. The answer depends a lot on other aspects of the boss and your relationship with him. I'd suggest thinking through the following questions to decide if you should try to gently approach the issue, or just move on:
- Does your boss respond well to constructive feedback?
- Does he try to make changes when a problem is identified?
- Do you have a strong channel of communication with him?
- If this were about another employee rather than him directly, would he respond reasonably to such concerns?
- Are there others in the company you can go to with your concerns?
- Is there a precedent in your workplace for employee feedback on working environment or company policies and practices?
If you can think of a less significant situation that has been handled poorly or ignored, I wouldn't hold out hope that this one will be handled well. I've been in situations before where I've tried to address issues but the truth is that the management just does not have the skill or motivation to deal with complex and sensitive issues such as the one you describe, and I've never doubted that moving on was the right thing to do.
However, if the company is reasonable in other respects and deals with smaller issues well, and you have a good relationship with your boss, then it may make sense to try to discuss your concerns with your boss. Perhaps broach the subject by first stating your admiration and respect, and then gently expressing concern that something is amiss because recent behavior has been so inconsistent.