Who's a what?

So, we just finished watching Beauty and the Beast (not for the first time mind you). But I was curious about who they had sing the theme song.  I waited until the end and once it popped up I said, "Oh, Celine Dion sang it."     Blake, who is sitting right next to me, responds with "Daddy, who's a Ding Dong?"

Where goes he get this stuff?