I'd say the best way to approach this situation is to follow his lead. Basically tell him to go first and then reply to him honestly in a way you think is sexy. I think a lot of girls worry that they'll sound corny or scare the guy off but the truth is he's likely just incredibly turned on you're talking to him that way in the first place. Talk about what you want to do to him and him to you and don't be afraid to be really specific. Think about it as if he were there with you. How would things progress? What might he do next? Tell him what your reactions would be physically. And if he says something and you're just not sure what to say, tell him how much it turns you on.