Most Helpful Girl
I know it's hard but the next time he comes in (whether he's alone or with someone else), I'd just casually say something like: "Hey, I just wanted to let you know that there was one time you came in and I made a joke but after I said it, I realized it might have sounded kind of like sarcastic but it really wasn't. And I noticed you didn't seem as happy after that. I'm sure you've probably forgotten about it but I just wanted you to know."
You'll feel so much better after you've said it !!
I'm only speculating but;
he may have had someone else in his life who would come across as sarcastic and it was just a bad reminder. It might explain why he didn't seem happy when he came in with a coworker (the first time). His coworker may have insisted on going even though he didn't want to.
He may have mentioned a girlfriend to let you know he's "moved on". He may not even have a girlfriend. ;)
Most Helpful Guy
How about you please help us by not having all your words smashed together in one giant glob?
I don't think he was purposely leading you on just to trick you. I think he was just being very friendly and had no intention of taking things further. However, if he breaks up with his girlfriend, you may have a good chance of developing something with him.