What if Nick started calling one of those other teenagers white trash or a wop or dago, laughed his head off and kept saying it over and over again? It's not appreciated when a white person says it so how do you think it would go over if a black kid said it? Not well, I assure you. The issue with the N word is that, as a teenager, Nick is disinterested in EVERYTHING. It isn't that he doesn't see color, it's that it hasn't been a problem for him in his young privileged life so it's irrelevant to him.
And trust me, Nick was not "ok" with the other kids calling him a ni**er. When he revealed he wasn't white, he suddenly became the center of attention and he loved it. When the N word was introduced, he was trying to be cool; keeping up the front that nothing bothered him - not even that. Once he ok'd the word though, he had to stick by it and the other kids happily jumped right on it. I mean, how could Nick say in one breath that he doesn't see color and the next be ok with kids calling him a reprehensible name that is only applied to black people? Nick has a lot to learn and, once he leaves his parents home, society will teach him about real life very quickly.