While I agree with Gentlewaters, may I suggest that as much as your bottom is going to interest him, it's how much you can teach him about his own sexual response that is going to change his mind. Asking him to do something to you that he considers to be "dirty" in an unsanitary way is not likely to float his boat anytime soon.
You're not only dealing with his desires and prejudices, there is a male ego that can be threatened by an implication that he might be gay (what his mind may question subconsciously). He may not even be aware that he is threatened subconsciously. No offense is meant towards him, but he might not be very mature in terms of his own sexuality and you could help him plant his own flag in this new world he might discover for himself (with your navigation and assistance, of course).
It's quite possible that no woman has taken him around the world before, that's to say, used her tongue to circle around his freshly-scrubbed anus. If he is excited by this, after several times of employing this stimulus in foreplay, a well-lubricated finger finger (wear either a glove or latex finger-cot) can provide the same stimulus: initially, at least, do not penetrate his rectum. In time, he might allow you to slowly push that fingertip into his rectum -- but limit that penetration only to your your first digit. In time, if all goes well, he might allow you to massage his prostate: when that happens, using various toys to massage his prostate might let him allow you to give him his first small enema.
All this is a matter of taking small steps. So to start out, the next time that the two of you are in the shower together, work up a good lather with both your hands; then with one hand, explore and play with his penis and testicles; with the other hand, soap up his anus (if you have sharp or long fingernails, this might not be a good idea and in any case, wear a latex finger-cot that you can buy at almost any drug store). If he pulls away, you've got your work cut out for you. If he doesn't pull away but asks what you're doing, tell him that you want to take him around the world: if he wants to know what that is, tell him that you'll "explain" this later... and if he doesn't ask what you're doing, take it slow anyway and don't make the mistake of penetrating his rectum. All in good time...