Boyfriend doesn't want sex?

My boyfriend and I (both 19) have been together for nearly a year and have just started having sex. We're together about every other day and I'm almost always the one who suggests "getting frisky" (as he likes to call it) and recently he's been refusing more than ever before. This might be because during the school year we're in a long distance relationship and only see each other about once a month and are of course all over each other during that visit. Now, during the summer, he sees me more often and perhaps wants more than just sex, which I am very willing to give. Could there be another reason why I'm almost always more horny than he is?