Problems staying hard?

My boyfriend and I have been having sex for about four months now. It all started well, but over time he has become unable to stay hard for too long. I don't think its an attraction problems because he says that mentally he really wants to, and I can tell that he is really frustrated and upset. He is in his early twenties and other than the fact that he has been a little stressed about finances, I don't see why he would have this problem. It's not like he's going broke, he just needs to pick up more hours at work (and he worked it out with his boss). Does anyone have any explanations.