M.L.
I think it's sad that Niki assumes that he isn't being taught this concept at home. Everyone has to blame parents for kid's annoying behaviors. Really, maybe he is being taught about it, maybe he isn't. But all you can do is keep firm about it. Really, it sounds like pretty normal behavior to me for his age.
Just explain, the toys don't belong to him and he doesn't need to be getting new toys all the time anyways, just tell him that straight up. Shoot, I would tell him that his constant whining for toys is annoying and he will get one when he least expects it as a reward for good behavior.