Depends whether you are interested in him or not.
If you are, then consider what everyone said about possible harrassment charges if it goes wrong. Is that worth it?

If you are not interested, you will have to tell him such things are not on.
I think it's best to be kind but direct and firm so it's clear he can't do this kind of BS.

You say he's embarrassed maybe, so what? If an adult can't control his urges ... His brain was clear enough to know it was inappropriate. If you do nothing, he'll think he can get away with murder. Might do the same thing to others, a female employee if there are any.
You also don't know if it's even genuine. Could be he was having a good laugh with mates, maybe they dared him into doing it.
I'd say let it slide, but do give him chit for it, in a nice way, but still in such a way he knows he's been chat on, lol.