A bottom that doesn't like his ass eaten...?!
Anyways, here are my top two reasons:
1) He just may not like having his groceries ate. I know, I know, I know it sounds crazy (esp. to me as a bottom 'cause I didn't put in all that work for you NOT to eat this bussy), yet it very well maybe something that doesn't get his engine running. Strange hang-ups people have these days, but we all have them.
2) This very well may be the reason (and he certainly wouldn't be the only bottom who fears this). He maybe scared of you "going down" on him and coming back up to kiss him all in the mouth with something er...uhm..."unpleasant."
LOL I'm just saying. Maybe coming up with some sort of bargain will do the trick? You do something for him and he let's you tickle his taint? He may not absolutely love it, but I've come to learn that sex, just like a relationship, is about compromise. When I was with my ex, he LOVED fingering my asshole, but I didn't/doesn't turn me on at all. What did turn me on though was seeing his face and the enjoyment he got out of it. It's the only reason I put up with it honestly...
i love your replies. honestly.
Anyways, hope I helped.