when my boyfriend asked me that- its only been once mind you. all I said was I wanted him inside of me. it was quick and to the point. so, if that's what you're thinking about go on that..
Tell him stuff like what you would do when you're with him. like how you'd straddle him and start kissing him and kissing his neck and then how you'd take his shirt off and kiss his chest and down his stomach to his pants... you get it? stuff like that. and continue from there. haha That's what I do to get my guy turned on. good luck.
My suggestion is to ask HIM what would He do to turn you on. It doesn't sound to me like he has much respect for you if he's asking you to turn him on. I mean, if he had feelings for you, shouldn't he already be turned on?
My feeling is that he doesn't really care for you and he's taking advantage of the fact that it must be obvious you have a thing for him. Of course, I can't guess what his true feelings are so I am just saying, if I am interested in someone, I am going to try to make all the right moves in order for her to notice me and get to like me, not the other way around.
Hope that helps,
Be honest, tell him exactly what it is that you would do to him, if you mean it then it will turn him on, but make sure you mean it because he WILL call you out on it.