If you live near a military base, going to the PX or BX would be a good first place to look, if you don't live near one they may sell Army merchandise at Target or Walmart (usually around memorial day, though).
Also, you can buy stuff when when you go see him graduate, there's a ton of places and things to buy around the boot camp areas. When my brother graduated from the Marines we bought shirts, bumper stickers, scrapbooking decorations, coins, all sorts of stuff. It was a little indulgent.
