My boyfriend wants to go into infantry, but he is very skinny/scrawny. Would he make it?

My boyfriend joined the USMC in hopes of becoming an infantryman, but he is still tall and skinny, and I don't think he has the brawns to make it, because those guys are the big, beefy warriors that do all the heavy work. My bf only weighs about 140 pounds, and I'm scared that he won't be able to do that kind of work. I heard the infantrymen run around carrying 90 pound packs and all sorts of weapons that weight anywhere from 10 to 100 pounds. Sure my bf works out a little, but he isn't that strong yet. Should I be concerned or worried? I don't know of any other guy who joined the infantry that was as skinny or lanky as he was, and most of the guys I knew who did that route were pretty big or muscular to begin with. Do you guys know of any infantrymen that started out like this? Does he have a chance to make it?