Tickle his asshole while you're going down on him to get him used to you being down there. Don't go in, just around the outside along with the balls and the taint. When he trusts you, maybe just push it a little bit (don't go in, just push). When he's ready, get a little lube and just put the pad of your finger in, then the first digit. This is the method I used with my wife and she's basically a butt slut now. A wise hooker taught it to me when I asked her to teach me something to please women that most dudes can't do. I used it on all of my girlfriends since that wise hooker and made them all enjoy buttplay. Nowadays, my wife takes twice as long to cum if I'm not playing with her butt.
I think I got off on a tangent. Anyway, back to the original point. Once he relaxes playing with the outside should be enjoyable. It's enjoyable for me and I've never had my prostate touched.