How to guide my boyfriend into a less embarrassing kind of 'goth'?

Ah I hear you, he's opting for the mallgoth/poseur look without any inclination about the gothic subculture whatsoever. I'm afraid there's nothing you can do about it other than to tell him straight up. Better he hear it from you then someone else. Maybe he will realize how much of an idiot he looks but I'm sceptical of that.