How old is your little cousin? What have you been teaching him, it sounded like you guys went through a lot of fun? In fact, it wouldn’t be a surprise at all. Children nowadays grow up and mature faster than how we are used to be. They have better exposure compared to previous times. Now there is information everywhere. Television, internet, etc, and most of the stuff they can get through internet, so it is not surprise at all that your little cousin would sing such songs.