Hey Fizzgig -
NTHEORY - I turned 18 in Navy boot camp in Orlando, of all places. I highly encourage your son to take at least 1 year of college, junior college, tech school, whatever you can talk him into, before he enlists. My son wishes he had had at least some college credits under his belt before he enlisted, and I'll bet any Marine will tell your son that, too.
As far as the passports, if a military member or government employee is taking a trip on their own personal time, they are just another tourist. Now if it is military or government orders, they will be issued a military passport if necessary. BUT the military passport should not be used for personal travel. In some countries the government ID card is enough; in other places, not having a passport would be a problem.
My son wants to go to Ireland. I have told him for years that I would pay half if he'd pay the other half and get his passport. Maybe one day we will go there together.