Could be because of his inexperience, his metabolism and/or his excitement from you. lol! From all what I've read and heard, guys vary from being very fast ejaculates to very slow. Some in a few minutes and other for much longer. It doesn't seem to have much to do with the physical stimulation for some guys. Of course his attraction to you helps too and that you make him that horny, but it also depends if this is a once occurrence or like ever time. He may not have learned to control himself yet, or you just get him so excited he has to just let it go. haha!
it depends on a lot of things, including how experienced he is and how long you've been together. if he's young and/or inexperienced and it's your first few times together . . . yeah, he's probably TOTALLY turned on and also nervous/over excited.