If he signs a contract before he is 18, he is signing up for the Delayed Entry Program (DEP). It is a commitment, but it is not actual enlistment, so he can still get out of it before the enlistment papers are signed if he changes his mind (the recruiters will protest loudly). The day he leaves for bootcamp, he'll go to the local MEPS processing station for medical evaluation, sign the actual enlistment contract, and then stay there over night before leaving for bootcamp the next day.

If he wants a particular job in the Corps, he needs to make sure ALL FOUR numbers of the MOS designation are on his contract. I got a bit screwed on this point. My recruiter explained to me I was guaranteed LAN/WAN tech, but my contract only stated 28XX... which just guaranteed SOME job within the Communication/Electronics field. I ended up being a Ground Radio Repairman (2841), attached to an infantry battalion and humping a toolbox for my entire enlistment. Don't get me wrong, I enjoyed it and still cross-trained for the job I originally wanted, but it was a big let down when I found out what was happening, and way too late to do anything about it.

I'll disagree with one post above and say volunteer for everything, especially in bootcamp. He'll end up on a lot of shit details, but he'll also get to do a lot of cool stuff. My drill instructors got their squad leaders for the platoon by asking for volunteers without saying what it was for. The squad leaders got promoted right out of bootcamp. So volunteering sucks sometimes, but sometimes it pays off too.

I will agree that being an officer sucks a lot less than being enlisted. If you can talk him into it, get him to go to college first and join the Corps afterward as an officer if he still wants to do it. He'll make a lot more money, have better living conditions, do a lot less shit work, learn excellent management skills that translate better to civilian employment, and generally have a better time.