What should I do if my loved one is bipolar and won't take his medication?

It's essential to understand that his motivations for not taking medication are complex and important. It may be that his medications have side effects that are uncomfortable. Possibly the medication he takes alters his sense of self and he doesn't like the new self that he lives with. It may also be that his daily dose of medicine is a painful reminder of his bipolar condition and he'd rather not face that reality. All of these are common reasons why medication compliance is inconsistent.
It's also important that you find some empathic connection with him around his choice to not take medication. If he feels you understand his experience of medication, he's more likely to be willing to talk with you about the issues and through that process, possibly find some flexibility. Conversely, if he feels that his medication compliance represents a capitulation to your needs and wants, then you're likely to encounter more resistance.
Taking medication for many with bipolar disorder is a mixed bag. It can often take a fair amount of time (often several years) for the bipolar individual to value mood stability with sufficient strength to offset the reasons underlying treatment resistance.
For the partner, spouse, friend or family member who has to stand on the sidelines and watch the bipolar individual suffer multiple relapses due to medication non-compliance, the experience is painful and frustrating. The key to managing bipolar disorder within relationships is open and honest communication where the issues can be discussed with a sense of mutual cooperation and respect.