He tells you to just "let it all happen". There is no concern for you whatsoever in that statement. Does he even care how his actions affect you?

Also, there was no mention of treatment. Is he on medication? Under the care of a psychiatrist? Does he take his medication consistently?

The fact of the matter is being hypomanic can be fun and exhilarating for a person with bipolar. Hypomania can be fun, until it's not. If he is someone who "enjoys" his episodes, life with this person will always be difficult. Just ask Kim Kardashian.

Once he is older and riddled with debt from untreated episodes or loss of all friends, he may come to realize there is a heavy price to pay for these episodes. That could be 20 years down the road.
And every episode will take a toll on you as well as him. You can't help him if he doesn't want to help himself.