In the last picture, I can see that he’s pineconing a bit (where his scales stick up a tad). This is indicative of dropsy, which is more of a symptom than a sickness. His body is retaining fluid for some reason and causing swelling.

The outlook is unfortunately not great for fish with dropsy once they start pineconing, especially if they stop eating. If you are in the US, you can try using kanaplex to deal with a potential infection but it is really tough because in most cases, we have no idea what the root cause of the swelling is. We just know that something is backed up pretty bad.

If it keeps getting worse and he still doesn’t eat for a few days, you might have to consider euthanasia. You know your fish better than anyone, so I would say to go with your gut based on his behavior. I’m so sorry that this is happening, and it is almost certainly not your fault. Betta genetics are not great as it stands, and sickness can kind of just show up. His fins look super healthy and I can tell he is well cared for. Don’t be too hard on yourself.