As a fellow massive anime fan, I think your boyfriend needs to realize that Japanese women often aren't accurately depicted in anime and hentai at all... Let alone any other nationality or race.

Hell, it's even a problem with dudes who watch too much American porn. They assume that women, in general, would be like pornstars in the bedroom.

Sorry, bud. Chances are that your girlfriend won't be into calling you "step-bro" or "onii-chan" while you're piledriving her on you kitchen floor.

Anywsy, just be as honest and open with him as you have been in here, OP. If he listens and respects your feelings... Awesome! If not, it's his loss.

Also, I'm all for trying out new kinks, roleplaying, erotic cosplay, etc. However, as with all new kinks/experimental bedroom play, it really should be discussed beforehand.