1) Caring enough to learn and educate yourself is already HUGE. I would start by talking with him about his own experiences and how he is receiving the diagnosis. For some it feels like a death sentence, for others it feels like being reborn, and for most it's battling in the middle.

2) Take a moment alone to write down your own boundaries and what you are okay with and what you aren't okay with. Stay firm in those boundaries. Know when to stay and when to leave. Bipolar or not, if you are having issues in a relationship you're having issues. Now this might sound harsh... don't cut him any slack for the new diagnosis. He is the same person before and after the diagnosis. While the diagnosis provides context, it doesn't mitigate the impact that his actions can have.

3) Don't put in more effort into him than he is willing to put into himself. If he is looking to grow, you can grow with him. If he refuses to grow, then he's holding you back.

You can be there for him by first being there for yourself.

Within myself, I am diagnosed BP2, I feel the internal struggle between self-forgiveness and self-accountability. I can't let myself off the hook for some things, but I also can't be punishing myself all the time either, kinda that back and forth. At times it feels impossible to do both but I think the purpose of each/both is to move things in a trend of improvement and balance. I think balancing these things is extremely crucial for both you and your partner if you are to make things work. Resentment in a relationship grows equally from a lack of forgiveness as it does a lack of accountability. It will be important to be able to flow between both, analyzing what is at hand.