All relationships ebb and flow, especially long-term ones. Stress plays a HUGE role in how much sex there is in a relationship. My SO and I burn up the sheets when stress levels are low; when they are high, sometimes it feels like we're just dialing it in and getting it done before we go to bed, if it happens at all. We do a good job of talking about our relationship (metacommunication) and discussing our own needs/wants/desires. One of the critical things we've discovered over time is the importance of taking time together to destress, unwind, and check in with one another. My SO is a senior in Chemical Engineering, and his schedule right now is INSANE, but he always makes sure that I get time with him on the weekends. Generally, that down time together leads to some afternoon delight
But it wouldn't happen if we were both still stressed out about school/work/whatever, which is what I think is happening with your SO--he needed to destress, he has, and now you're going at it like knives again. Just bear that in mind for the future, and you'll be set!