What's a good way to discuss with my bf that he has narcissistic personality disorder?

Depart Forever: This is a serious disorder that will destroy your life if you stay with him. Do not be the therapist yourself. Depart now or be destroyed. Reference site: http://psychology.About.Com/od/personalitydisorders/a/narcissisticpd.Htm.

Psychiatrist: Have him see a psychiatrist to make the diagnosis. It will be more reasonable to have a specialist verify and recommend therapy.

You can tell him but: Either he won't comprehend what you're talking about, or he'll be narcissistically wounded and enraged at you for insulting him. So, i wouldn't say such a thing to anyone who has not invited me to offer a diagnosis. And unless you're a mental health professional, that's out of your area of expertise anyway. You can consider getting help for yourself, also whether you want to continue with him.