Well being military, and having done the whole promise ring thing before. In my honest opinion, if you want to get him something to think of you while he is gone, get him a necklace, nothing tacky, but nothing too masculine.
Yeah, personally, I don't like promise rings myself. But that's just me though. Find out if he'd like to wear a necklace. Perhaps a watch or something. I don't wear any other jewelry other than a watch myself, so its up to your guy what he likes.