I carried a variety of knives, but a becker bk11 is a great little utility knife that is hard to mess up with. it will open your mre's but its thick enough to snap ammo bands with. If your willing to spend a little more bank, the ESEE-4 is pretty much perfect for anything he might want to do. It takes up little space and adds little weight and its beefy enough for most any chore.
He will be issued a multitool. and most likely a folder so i would stay clear of those things.
More than a knife though which he can purchase what he wants, i would send comfort stuff like wet wipe packs, 4 hour energy, weight lifting supplements, tobacco and pornography. just imho