My mother is bipolar. And as has been said, it is in fact part of the illness that they go off of their medication.
If you can imagine feeling completely elated and full of energy and and even super power like, like God. This is mania. Then follow that by deep severe depression. That is the depression side. Hence bi-polar (polar opposites) or manic/depressive.
For some reason when on medication they feel restricted. I mean, no one wants to feel like they have to take medication to be able to survive. But unlike the many other diseases out there where people are so thankful to access to medication (diabetes, cancer, hypertension) the bipolar individual looks at the medication as an evil. Not even bringing up that if you try to convince them that they are ill when they are manic they think YOU are crazy because they feel so damn good!
I am tied to my mother because she is family. As a matter of a fact we in the past year built an inlaw apartment for her to be able to live with us so that I can more closely monitor her. It is so unbelievably taxing to take care of someone (especially when you have no control over what they do) who is mentally ill.
People with this disease frequently end up divorced, homeless and suicidal. You do not have an easy road ahead of you if you are going to commit to trying to help him.
My advice, try to contact his family. Bottom line is he needs to be on medication. Offer to go with him to his doctor, or at least drive him there. Be prepared for him to be upset with you for suggesting it. Do not over extend yourself to help him, and understand that you just may not be able to.....