I wouldn't try to discourage him either; he's a big boy and should make his own decision. There are some books about war that he should read first though.

'And No Birds Sang' by Farley Mowat is the story of his service in WWII. It's extremely well written of course and the first part is lighthearted, very funny in places. It gets a lot grimmer once the killing starts. Everyone thinking of the military should read it.

If you can find it 'War Year' by Joe Haldeman is a fictionalized version of a grunt's year in the Nam. Haldeman was there though and little of this book is very fictional.

There's may others of course - All Quiet on the Western Front, Catch 22 etc but I think Mowat and Haldeman are the best for a current young person.