First of all, I'm not 100% sure he's depressed, but he refuses to speak about what's going on and gets quite angry if I try to talk to him about it. He has been a lot more irritable and quiet lately so I'm 90% sure he is depressed. He has also struggled with depression issues in the past. 

He has also been sleeping a lot, showing less interest in the relationship, is a lot less talkative, and has shown 0 interest in sex for a couple weeks now. This sounds like depression, right? 

I want to help him, I just don't know how to (even though I struggle with bipolar II so I'm very familiar with depression myself). He has completely closed himself off it feels and it's hard to not take it personally. I thought he had checked out of the relationship before he told me he was "going through something" (but again, won't say what and I'm not going to pry). 

How can I help him?