I have some experience on both sides of this. If you have a question about the specific military school you may even find that some of this board's posters are on the staff.
- Barterables like gourmet coffee (ground Kona or Jamaican, the expensive stuff), small candy bars (like the bite-size Hallowe'en bagged ones), powdered energy drink mix (Gatorade, Powerade). Crystal Light drink mix.
- Barterables of a different nature (not the sort he'd want to get from his dear ol' Mom) like a subscription to Maxim or FHM or Outdoor magazines, Nicorette chewing gum, chewing tobacco, fine cigars. (In other words, testosterone & nicotine.) Best to check out the school's regs first to see if there's a potential problem, but he might be willing to risk that anyway.
- Speaking of testosterone, "inspirational" posters. These come in two categories-- the ones put on a bulletin board for an admiring public, and the ones hidden behind a closed closet door for just him and the roommates to admire.
- Magazine subscriptions that he might enjoy for a hobby or a sport. I'd recommend a financial magazine, too, but most of them are crap and "The Economist" might be a little over the top at this point.
- "Local" candy. For example our Hawaii-grown kid has left detailed instructions to send bags of lychee jellies, shredded coconut, li hing mui mango, li hing sour watermelon, and li hing mui powder (in the handy five-pound sack). She even gave us cell-phone pictures of the bags hanging on the store's candy rack.
- Jerky-- beef, turkey, pork, other "interesting" flavors.
- Canned foods like SPAM. Sometimes you need calories but you can't afford to be seen in public.
- Power bars. Higher protein is better but the chocolate and peanut butter flavors are yummy.
- Dried fruit. Sometimes a quick sugar surge is essential.
- Coffee. He might have access to a coffee machine but instant would probably be preferred. You might also want to include sweetener like Splenda or Nutrisweet and powdered creamer. No vanilla or other fruity flavorings unless he's willing to put up with the roommate/upperclass abuse.
- Depending on the climate, powdered hot chocolate.
- Teabags. Earl Grey or green or herbal.
- If he has access to a microwave oven, then microwave popcorn. You could include furikake or other flavorings.
- Cookies should probably be home-baked, preferably chocolate-chip, ginger, sugar, peanut butter, or oatmeal. Try to stick to the classics so that everyone can share. Or he might have a favorite flavor that he can keep all to himself.
- "Little Debbie" brand snacks used to be a huge hit when my generation was in college.
- "Soup for One" or "Cup o' Noodles" or even regular ol' ramen.
- Oatmeal, preferably the little flavored bags, or cereal in the little variety packs.
- Funny coffee/tea mugs. A cereal bowl/silverware. Maybe even a small submersible electric coil for heating the water in the mug.
- Tupperware containers help keep out rodents, bugs, and suspicious sniffing upperclass.
- Cool/funny t-shirts. The more local the better.
- Cool/funny signs or bumper stickers for the bulletin board or desktop.
- Paperback books, graphic novels, or comics that he's particularly fond of. For example Jim Butcher's "Harry Dresden" books might be tough for his school's bookstore to keep in stock and you could pre-order it from Amazon.com for him.
- If he has access to an MP3 player then an iTunes gift card.
I don't know how much money you want to spend on this support, but my nephew the Army Ranger was really really really happy with his Kindle in Iraq. And a set of spare batteries.
You might want to pad the container with a local crumpled-up newspaper. Yeah, he can probably read it online, but it's kinda nice to smooth out the sheets and reminisce over the local fire/police reports.