Calling your SO "Daddy"?

okay, so this isn't really an opinion thing, i just want advice. My boyfriend has asked me to call him Daddy during sex but this was last yearish and i was too shy at the time. I don't really want to bring it up in conversation, i kinda just want to suprise him. What could i say and when? Lol I need some examples. I understand if you think it's gross or whatever, but i'm kinda into it so please help 😂