I can only realy speak of the Navy.
Travel is the number one benny of being in the Navy. I got paid to go places people save thier entire lives for just to see one of them.
There are a vast range of jobs (rates) that he can do. Also if he's unsure, he can sign up as a "Striker". this give you the chance to look into an array of rates before you pcik one. I don't fully recommend it, b/c strikers get used and abused until they pick something.
Ask how quickly rates promote. Some are full and advance slowly, some are in high demand in the civilian world, so promotions happen quickly.
Remind him that Bootcamp is 8 weeks and 8 weeks only. Navy boot is more of a brain game than a physical one. We don't march around with packs, but we need you to react when the ship is on fire, or flooding.
Being on Subs was part of the best times of my life. A small crew where we knew each other, and relied on each other.
If he wants to meet chicks; Corpman, not yeoman is the way to go. (nurse, not secretary)
PM me if you want more.
What's a corpman?