Well... I dunno if your boyfriend is into gaming but a game might be a really good idea if he is... some dudes spend as much time playing games as they do with their girlfriends if not more (but then again the gamer types don't usually have much of a social life so..meh). Anything that he can carry round or use often, and..... can be disposed of easily when you break-up...jokes
Get him a necklace or some kind of shirt anything that he can carry around with him a lot he will love it no matter what cause guys don't expect girls to really get them anything. So when they so they want to have it with them all the time