Men hit on me when I was hubs's girlfriend, when we were engaged, and since we've been married. I don't tell hubs about each time; not even when it's his friend/acquaintance. Partially because I'm pretty clear that they will never get anywhere, I'm firm, and it never comes up again.
As a side note -- Men are pigs.
My question to you is. . .what are you confused about? Frankly, it sounds to me pretty clearly that he was taking a last shot at getting in your pants, and is now trying to backtrack. It's up to you to decide if you want to let him. I'd probably be quit direct, as Myra suggested -- "Jon, I want you to know that what you said made me pretty uncomfortable. You should know that if it ever, ever, happens again, I will tell my fiance, i.e., your boss, and your girlfriend as well." And after he blubbers, "it was nothing, a total misunderstanding", I'd say "Good." and then I'd drop it, completely.
Put it this way -- even if he's unclear with you, you need to know that you've been only completely clear with him.
"I'd hate to advocate drugs, alcohol, or insanity, but they've always worked for me." Hunter S. Thompson