I suggest waiting until he does it again, then making a light joke, but not one that is mean. So like, not "still looking at yourself?" because that's judgmental, but maybe "checking for grey hairs?" or "looking to see if your ears grew?" And see how he takes it. If he gets the hint that he's spending too much time in the mirror, you're good...if he doesn't take the hint...might need to actually directly address it. You could also just assuage his insecurity when he's doing it by saying "sweetie, that's enough inspecting yourself in the mirror, you look amazing. I am so happy to be seen with such a good-looking guy." Yes, I know that's kind of schmaltzy, but even men like to be complimented.

Seesaw