R.M.
This isn't very "useful" but what my mom got my husband when he was leaving for boot camp was a St. Christopher medal to wear on his dog tags. He is the patron saint of travelers, to keep them safe.
Otherwise, any care package type item is great. Barracks are similar to dorms, so any item that would be useful to a kid going away to college would be good. (Obviously you don't want to buy him a hot plate because that wouldn't be fun to put in his duffel bag. :) Prepaid cards for anything are always in demand on deployments. I love all the ideas that Dawn lists. A deck of cards, some DVD's, an iTunes gift card, some magazines or video games if he is taking a game system. Entertainment is probably the biggest thing for this type of deployment.