I know he is a narcissist and wants to keep it secret, but he could just lie instead of admitting it. =_= How they start their relationship is truly dumb and unrealistic.
Moreover, I don't think setting your selfie pic as wallpaper is something considered bad or narcissistic. I take 100 pics of me if I'm in the mood of taking selfie, and if there're some good pics, I'll make them my wallpaper. Dude, it's just a pic you like, and you want to see it whenever you use your phone. It's for your convenience, and seeing something you like also makes you have good mood, too. The end of the story. Why do you have to worry about it?