i'm sure your guy will change if u do have a baby.. and right now.. just think that's he's so relaxed being by your side that he can't help but fall asleep
Maybe he's just tired. Does he work alot? My bf was like that for a while, especially because his job required him to be on his feet all day and sometimes he'd work over 40 hrs a week, so he'd sleep alot. If it continues for a long time, then maybe he should go to a doctor as it could be something medical.
