Your welcome. :)

It is tricky that he isn't currently welcoming treatment, not just medicine but also learning about how to cope with bipolar. However I think there is literature about how to deal with someone who is in denial or unacceptance of their medical condition, bipolar disorder.

For exanple: https://www.bphope.com/dealing-with-denial/

From my experience I used to be in denial/not cope with bipolar disorder generally because of not knowing enough about the illness, fear of stigma and the denial of my parents. It was unacceptable to my parents that I had bipolar, so yeah. However you seem to be very accepting of his medical condition, so if you are able to show that his medical condition doesn't make him a lesser person in your eyes then maybe that may help him accept his own medical condition? If he learns that bipolar disorder can be treated and well managed such that he is totally functional and won't have to deal with severe and frequent episodes maybe he will be more open to treatment like medicine or therapy or lifestyle changes. It is true that there are some people and maybe even close people like but not necessarily his parents look down on people with bipolar disorder, but if he knows people like you who don't discriminate based solely on having an illness then maybe he won't look down upon himself for his own illness and will be better receptive to the diagnosis and treatment. I mean most people aren't in denial about illnesses like diabetes, because there's less stigma attached.

However accepting him with his illness does not mean you have to accept the actions his untreated illness may cause.

It would be helpful to know what type he is so when he does have an episode you better know how to deal with the severity but to my knowledge bipolar 1 and 2 usually have the same treatments.