Good question. Although getting someone to do something they don't want to do isn't really cool, I think often times in relationships, especially long term relationships, many people get into the habit of not challenging themselves or their boundaries. With more long term relationships trust is often established to where we can nudge out partners out of their comfort zone, safely, or we should be able to ask for at least a discussion about this we want to try even though our partners might have reservations.
My partner and I enjoy prostate and anal play quite a bit, but it does take time and a gentle approach when first introducing it into your relationship. There are a ton of how-to posts on here so I wont bore you with talking about that aspect. I think you'll have better luck with creating a potential opportunity to play with your man's butt if you had a deep discussion about your sexualities, how they came to be, what messages you received about sex as children, how your potential faiths influence perceptions of what's "okay/dirty/sinful/ple asurable/taboo" ectect...By asking open ended non judgmental questions to really get a deeper sense of his reasons and how those were created, while also sharing in a deeper sense why it's important to you to at least try (if it's even a burning desire..) you most likely has a better chance of success, if not showing respect to each others sexuality and value as a partner
Plus, younger guys, if he is young, tend to be a bit more hung up on these sorts of things in my experience. Takes time to grow into your sexuality and comfort zone, so if now is not the time, check in to see if you can revisit the topic later down the path in your relationship.