Why is my 9 year old cousin so sensitive to swearing?

He snitches on anyone who swears and makes a huge deal out of it. When he hears a swear word in a movie or book, he'll tell me not to watch it because it has a "bad word". He totally freaks out. He likes a few Minecraft versions of songs but he refuses to listen to the original lyrics or any other songs besides those because the original versions are "bad". He also thinks the words stupid, sex, or boobs are bad words too!

His parents are raising him to be proper.

He was indoctrinated to thinking swearing is "bad" or wrong. He will change as he matures and hears his friends cussing and using dirty words.