We have the same issue with my DS. He had his 5th birthday party in April and he has not even bothered to get any of the toys he was given out of their packaging (next year we will be doing a "no gifts" party!). The thing he really loves is chess - have you introduced that? Other possibilities are some of the single-player games from ThinkFun (his favorite is Rush Hour), or puzzle/logic magazines/books. That's all we're having success with right now (and by success, I mean he'll do these things once in a while if he can't play chess for some reason).
