If we're going to talk about ways to euth a fish, IMO the most humane way is to use clove oil. You put the fish in a deep sleep with the clove oil and then net it and dip it in boiling water for a second or two. This is the method that I've heard of being most humane.
Honestly, I think you should euth him. The chances of him recovering are very slim, and I wouldn't want him to suffer.
But, it's ALL your choice. I strongly suggest you don't flush him though..