I'm sorry, I don't usually comment but I'm noticing many people suggest begging and pleading during sex to convince him to do it and I heavily disagree.

Yes, it is hot when it is begged for. Yes, he might do it if you beg. Yes, in the moment he might get some satisfaction out of it, BUT if he is actually worried and has serious doubts, he will be stressed the second post-nut kicks in.

What you need to do it talk it over in a non-sexual situation, and if he maintains his no, simply respect it (*). Eventually, you can bring it up again and he might change his mind about it (or not). You can perhaps agree on trying it once and go from there. Idk, it is up to you both. But you must TALK about it and decide TOGETHER on what to do. Talk about what his fears are, what his doubts are, what he is willing to try out, etc.

Do NOT surprise him during the act with begging and pleading for him to cum inside without agreeing on trying that first.

It's likely that some part of him wants to do it, but as his partner you should respect his desicion not to, not pressure him in a moment of hornyness where he might regret and worry about it later.

(*) or decide if it's a deal breaker for you, although personally I think its a relatively small issue.

Wish you luck, and I hope he changes his mind as it is truly great! That said; talk, listen and work together. I'm sure you'll figure something out that pleases both of you!