Boyfriend wants me to....

Ok so he wants me to like eat his ass. But like not go inside but just like rim job basically. Im totally up for trying new things he wants but personally id rather try fingering him first but thats a huge no for him for some reason. But anyways im not sure how to get to that point. It took me a little while to get comfortable with him eating my ass lol so idk. I definitely want to try anything he does but idk how to go about it. Anyone else? Advice?