[Vulture6]

A lot would depend on what type of unit he was assigned to.

MPs are Military Police.

If he is assigned to a base unit or Provost Marshall's Office (PMO) then he is a law enforcement officer on the base. This would involve everything from traffic enforcement, correction (the brig - military prisons) and basic law enforcement to base security (gate and perimeter security, special events, VIP protection, and special reaction forces).

If he is assigned to a Fleet Marine Force unit (FMF) then his role is focused on supporting the warfighting mission... from unit perimeter security and convoy security & control to handling of enemy prisoners and detainees.

The best way to find out what he does is to ask him. Is he still in school or is he already assigned to a unit?