Tell him to investigate his likes and dislikes. (Land, sea, air, flying, airplanes, boats, etc.)
The military will give him a battery of tests to find out what field of expertise suits his aptitudes, abilities and personality. The tests, as far as I know, are pretty standard throughout the military. (ASFABS ?)
Any old boatswain’s mate will know all about that stuff.
(As far as prior DWI, unknown.)
(Good luck and Godspeed.)
ASVAB. There’s even study guides at the bookstore.