How to deal with stress in a relationship?

OK so a while ago my boyfriend and I started fighting more and more. We didn't used to as much just little arguments nothing big and now its like we fight way too often, usually its about things that I can see could be a bit annoying but not worth fighting over. The thing is we're not like this. I really believe that the only reason we've been fighting so much is that he's got so much stress on him right now. Stress of lots of different personal issues but issues that I really don't know how to fix. A lot of the time I find that I am only mad or upset because he's upset about something so small, or he won't tell me why he's upset or he's upset for a long time while I get over things really quickly. A while ago he said I didn't try to see his side of the arguments and that made everything worse and stuff so I've bee making an effort to get his side of things, but when I ask he won't tell me or when I ask him specifically if it's something he won't answer. A lot of the time he just says he fine which is code for 'if you don't know what you did then obviously you did nothing wrong'. I love him like crazy and he's a really great guy but the stress is getting to him and I don't have any de-stressing ideas. Does anyone know how I can help him with his stress?