The decision to euthanize is not an easy one and I've seen a few bettas bounce back from what I was absolutely sure was a death sentence. But there also comes a time when everything has been tried and the fish is clearly in distress. Generally, the criteria I use to judge whether or not enough is enough is if the betta will eat, if they can swim at all, and if they are laying on the bottom gasping with heavy gill movement. The last one is the biggest indicator of distress.
If it was just his popeye and he was in pretty good spirits otherwise, I would say he still has a chance but the fact that he's not eating is worrisome to me. He can still pull through but his odds got a little smaller. If it helps you make a decision, fish don't feel pain and discomfort in the same way we humans do. He's not suffering the same way a dog or other mammal would in the same situation.
You will make the right decision, whatever it is, I know that much. If you do decide that he has been through enough, just let us know and we can help you through the process of euthanization.
No matter what, Tidbit has been incredibly lucky to have you in his life.