I wouldn’t let him off the hook by making a joke of it. I’d never want to set a precedent with a partner that it’s ok to lie to me. Where could that lead?
I’d sit him down and let him know that I knew about his lie, that it had hurt and concerned me. That honesty was crucial for trust, in my mind. Then I’d stop, and wait for what he says. I think it’ll be illuminating. If it’s been bugging him, and he’s been losing sleep over his deception and web he’s caught himself in, then I’d see that as positive.
If he just carries on in a defensive way, or tries to brush it off as ‘well everybody does it’ I’d be more worried.
Basically, his ‘little lie’ shows him to be dishonest, vain and manipulative (even though not in a serious way). The fact he hasn’t fessed up, shame-facedly, as time has gone on shows him to be a coward. And suggests he thinks you’re stupid, surely?
I’d definitely be asking how he thought this silliness would play out, if you hadn’t brought it up.
Again, I don’t see it as immediate cause to leave him, but his reaction to ‘The Talk’ might. See how much he acknowledges your feelings, and explains his own ethical dodginess. That’d be the clincher for me. State the bare facts, then sit back and watch carefully to see the measure of the man. If he responds like a little boy, then you have your answer.