My (24F) boyfriend (26M) struggles to express his feelings, how can I be patient ?

Im in a bit of a pickle. Im with a really sweet guy. Very witty, fun to talk with, good in bed, etc. Im a big fan. However, Im noticing he has trouble expressing his emotions, especially when something bothers him.

The best way to describe it is, hell bring up something that upset him, Ill explain my side/apologize, and he will continue to dig deeper and deeper and reitterate and rephrase the situation when, I understand and explained it and apologized.

Then he will say "it doesnt bother me though, so its fine." and then say I am overthinking the situation when I apologize. Last night we had a similar thing and I said "Im not, if it bothers you it bothers me, and youre upset and thats okay."

Ive never encountered someone flipping overthinking to me and being uncomfortable when an apology is given. I was pretty bothered the first time it happened, just because I was very confused lol, but now Ive largely figured the dude has probably never been apologized to and someone giving a shit that his feelings were hurt is new and uncomfortable for him so im largely unphased and try to just handle it in a healthy manner regardless.

Im trying to gently get him to understand that its normal for someone to apologize and its not overthinking in my part. And its okay to say something bothered you and not backtrack. Any advice ?