I'm assuming rifle hunt? I'd make sure he's comefortable with the rifle first and foremost, sounds like you're going guided/outfitted, so i wouldn't worry about too much than being warm/ready comforatble, let the guide lead and have the son ready to shoot. I went on my first big game hunt for antelope/mule deer when I was around that age. I certainly needed more rifle experience. The rifle at the time was scary to me, kicked way to hard, I was scared to shoot it, I also was 5'2 and 110 lbs. so speaking from my own experience is practice practice practice, try to offhand shoot if you can, even at 50yds, I'd recommend a comfortable smaller cartridge even if on the light side, nowadays they have reduced recoil rounds too. Also for those long shots, practice those off the bench, and off some shooting sticks, maybe pick up your own and learn to use them. don't want to be on your hunt about to shoot the first time you actually use them. Also be in shape? stay hydrated at altitude......go slower than harder day 1? talk to the guide before the hunt, now, also before you get to camp, pick their brains, so when you get in the field nothing is assumed, i remember, haha, day 1 of my first hunt, on horseback my father and I, he actually hadn't been on a big game hunt out west either, a nice mule deer buck gets up, the guide hops off to check him out, looks back, and we're both still on our horses! haha. says to us that's not what you want? turned out to be about the best /avg buck offered, we were naive, wanted a monster, and didn't know the drill, so figure all this out with your guide, ie the drill, what to expect, so if you see that 5x5 first morning and its a monster for the area, take it! not expecting somethign that simply wont exist there. figure all this out ahead of time.