Dating someone with bipolar depression : /

My boyfriend has bipolar depression. I’ve never dealt with this situation before and I don’t know how to react to it. We go through phases where everything is great and he shows affection really well but then he’ll switch up and be upset because he feels he’s not good enough. He just sits in his room and sleeps and cries. I think his family is really toxic. His dad makes fun of him all the time and his mom is super toxic. I don’t really like them. He’s constantly baby sitting his younger siblings and has no time to himself. He’s very open to me about when he feels this way and I’m glad. I would never leave him over this I just don’t know how to handle it. He said he just needed someone to talk to last night but I kept repeating myself. Saying how he makes me feel amazing and how much he means to me but he would always reply with something along the lines of him not being good enough. I don’t know how I’m supposed to have a conversation with someone who won’t believe a word I say. How am I supposed to help? I want to be able to help him but I honestly just don’t know how. I love him so much and don’t want this to get in the way of things.