Boyfriend Ass Play

Soooo I have a kink for my boyfriends ass but haven't really tried anything and he doesn't really know ! Well he loves when I touch and grab his ass or kiss it.. And I tried to like touch his asshole and stuff one time but he stopped me. Later I asked why and he just said he wanted to make sure he was all clean. But since then I've never tried anything but kissing, grabbing, etc. I really wanna give him a rim job or something! What should I do.. I don't think he's so into the whole idea