You could buy him something like a mix CD of his favorite songs or tunes, a nice photo frame with your picture inside it or a nice pen. You could also give your boyfriend a combo pack of nice smelling soap.
Since I don't know your boyfriend it would be hard to tell you something sweet. For my husband who loves pistachios, anytime I want to get him something small I get him some of them. You just have to figure out something he enjoys and get that. Is there a particular candy bar or something other thing he gets when he wants to treat himself?