Just start with things like “mmmmm” “ooooh” and “yes” to get him used to you introducing it in the bedroom. You can breathily whisper them into his ear at first. Then as you get more confident go with what feels natural, whether it’s a “harder”, “oh my god” or letting him know how big he is or when you’re gonna come. Key is keep it natural, you’ll both feel more comfortable and enjoy it more.