Being with someone who is Bipolar is extremely difficult. Especially if he's not being treated. People with bipolar disorder are always cycling between a manic state (high energy), depressive state (low energy) and a normal state, and it can be difficult to predict which state he'll be in at any given time. The fact that he's working two jobs and going to school is probably going to exacerbate his bipolar disorder. Things like stress, hunger, fatigue and anger can all trigger manic or depressive episodes, and if he's not careful to take care of himself, he could really get into a dark place.
Being there for him is all you can do, but you have to understand that it might not be enough. You can't be his therapist or make him feel better when he's down. He will eventually need a psychiatrist to prescribe him medication so he can stay in a normal state of mind.
I think the best thing you can do, is just to listen when he's upset. If he's not willing to talk about it, then you might want to let him know that you can see he's not in a good mood and let him know that you're willing to listen or do anything you can to make him feel better. At least, let him know you'd like to be able to help him feel better. Whenever my boyfriend is feeling depressed, I just listen carefully and try to help him by reassuring him that the things we're dealing with now are really hard, but things won't be this way forever and someday we'll be able to say we made it through these tough times by sticking together and helping each other out.
I hope this helps a bit! Good luck!
