I guess you're right to be a little down, especially if you're used to seeing each other all the time usually. Exams are stressful, especially the ones at this age.
No need to worry, unless it carries on after exams, but it doesn't sound like it should. I'm all tied up with exams and work at the moment so don't have as much time as I did a few weeks ago for my girlfriend, but if he's like I am, he will miss you, but will know exams must come first right now and he hopes you understand and are behind him.
Why not call him tonight when you/him usually go to bed. Don't ask for much of his time, just call to say goodnight and wish him luck or something. He won't be working just before bed, so you shouldn't be in the way, and if it's only short won't stop him getting the sleep he needs.