I think you should tell your 5 year old that Santa has always only existed for those who believe in him. He's magic like that.
You could also tell him that Santa's kryptonite is kimchi, but then he might be afraid to eat it.
I would bet the guy thought he was only doing you a favor by having your kids beliefs conform with what he knows as normal. In Korea Christmas is more of a couples / dating holiday anyway, right? Probably take a while for it to really catch on if ever.
