Take it slow and easy. Once he's on his back and ready, climb up on top of him and slide him into you gently, and for starters, let him thrust up into you instead of you pounding back at him. You can start with that once you're both comfortable with the position. It'll take some time maybe to get both of you adjusted, but it's a great experience to learn what works in a new position. Plus, with his hands free, he can roam them over your breasts and stimulate your clit once you've got a nice rhythym going. Or he can reach around and pull you tighter to him when he's squeezing your ass, the possibilities are endless! Maybe he'd even like to see you put on an oral show with one of your toys!