A good knife is a plus to have but that is probably a given.
Camelback is awesome for over there but he probably will have one issued to him.
If he doesn't have a poncho liner (and some national guard guys I know didn't) I would suggest he get one. I would also suggest having a zipper sewn into it to make it into a sleeping bag. This was a major comfort/utlity item for me when I was there. It seemed like no matter how bad things got as long as I had my woobie (poncho liner) it was going to be ok.
A ditch medicine type book might be of intrest to him.
Its been awhile since I was there so I am having to think about it.
An Assault pack. Blackhawk, Eagle, Tactical tailor, Camelback brand. Get one he can strap stuff to the top, sides, and bottom of. When I was there I lived out of my assault pack. My ruck only served as a resupply point for clothes.
Good socks. No make that awesome socks.
A platypus bladder might come in useful to him to carry additional water. If he doesn't like two quarts.
I am going to have to think about this some more and if I have more ideas I will post them. Oh, if he is getting the MOLLE system the woobie will pack into the butt pack from the molle just right (this item is a don't leave home without it item).
Wish him luck for me.