Why did my boyfriend stop cumming during sex?

Ok. I’ve read a few of the forums on women who have problems with their partners cumming in general. However, my case is a bit different. My boyfriend (21) and I have been together for almost 2 years and having sex just about as long. We used to have sex almost every day of the week (with sessions lasting approx 30mins or so); however, now we don’t get to do it as often due to work/school. We were having no trouble having sex 2-3 times a week and the sessions were getting longer (like 1.5hrs-2hrs with small breaks in-between). Much like some of the comments on here, he IS a pleaser–he does everything to get the job done with me. The problem is, I like being the same way–I don’t stop til he’s done too. Now, for the past three months, in the time we’ve actually been able to do it, he’s been taking longer to cum–I’ve asked him if its because he’s masturbating more and he says no (we’re very open about that so he wouldn’t have a reason to lie to me about it out of shame–I, myself, masturbate on my own too). Then, about 5 times in these three months, he hasn’t cum at all. We’ve had a couple of entire days to ourselves and while I cum an average of 6 times, he’ll only do so once (after HOURS of trying–yes, with breaks, of course) or he won’t cum at all. Now, we have actually TALKED about this–like I said, we’re open to these kinds of things w/ each other–but he says he honestly doesn’t know what’s wrong. He hasn’t watched more porn than he did before, he doesn’t do drugs or drink, his diet has been pretty much the same and he has no crazy stress plaguing him right now. I love him. He knows it and he says he loves me too. I know better than to make a guy feel ‘self-conscious’ about his abilities in the sack and I reassure him that maybe its just our change in schedule, school and all that. Nevertheless, I notice the concern in his face…like somehow he feels as if he’s let me down or something. I don’t know–any advice would be great. Sorry for the long question.