Wait for the Punchline
I can see why this dad likes to tell this story so much! If this was my family, I’d be going around telling this story to everyone I know. Anyhow, I don’t think this kid has anything to be ashamed of. We were all pretty dumb when we were 16, right? Some of us believed in Santa, and others said or did other dumb stuff. We shouldn’t be judged for the things we said when we were teenagers as long as those things didn’t hurt anyone.
And to be fair, saying “your mom and I are Santa Claus” is a pretty confusing way to say that Santa is not real. I don’t think his assumption is that ridiculous. He still believed there was a Santa – this just made him think that his mom and dad were the Santa who goes around on Christmas eve spreading joy and gifts.