Well every person is different. Depending on what his personality is like he might be very open to discussing things with you.
Bipolar disorder is a very difficult psychological pathology to deal with in a relationship. This disorder is characterized by the patient having a cycle of Highs (mania), Lows (depression), and normal states. Although, some patients don't have any normal state and just cycle between mania and depression. The disorder can be managed with medication, but what tends to happen is, the patient gets enough medicine into their system and thinks "Hey! I'm feeling better! I don't need this medication." So they get off their medication, and start having more manic and depressive episodes. This can be dangerous because you can't just stop taking medication like that all at once, you have to be weaned off with the direction of a doctor.
The thing you should understand and accept now, is all you can really do is to let him know you will always be there for him and support him. You cannot always MAKE him feel better, or be his therapist, or deal with his bipolar episodes for him. I'm not sure if your boyfriend is on medication, or seeing a professional at the moment, but it would be best if he was doing both. At least on some kind of medication. If his behavior is becoming a problem for your relationship, you need to discuss it with him. Let him know that you'll do whatever you can to help him, but he needs to take responsibility for his disorder. You don't want this to take over your relationship, and if he cares about you he needs to understand his behavior, and how to prevent it, so that he doesn't do something or say something to hurt you while he's in a manic or depressive state.
I had a boyfriend in the past who was bipolar, and it became a huge problem, because even though he knew exactly what would trigger his episodes he wouldn't work to avoid those triggers. He also expected me to just "sense" when he wasn't feeling good. So whenever he did have an episode, he would blame me for not reading his mind and making him feel better. He would get so depressed that at one point I had to hand feed him! You don't want to get to that point. I started to feel like I was crazy! When I brought up that he should try to find some help, because he had no therapist and no medication, he didn't make any effort to get help. I went to therapy and that's when I figured out that I was the normal one, and I needed to get out. Eventually I did, but it wasn't only due to the bipolar disorder.
If you want to discuss this a little more, feel free to message me! Good luck! :)