The N-word

Okay so, I'm black and my boyfriend is white and he feels it's okay to say the N word. I haven't outwardly voiced my opinion to him, but it does make me uncomfortable. well, the other day we were in his bed and I forgot what we were talking about and he called me the N word and it took me a while to gather my bearings. his excuse? "it was said in GTA"... I don't know how to bring this up without making out seem like a big deal...