Let nature take care of it.
If he is in a lot of pain and you dont think he will make it and hate to see him in pain. You can put him in a bucket which is safe from chlorine and then add some clove oil keep adding it until he is swimming at the bottom of the tank and his fins arent moving much then pour some vodka in which is poinous to fish this will not hurt him as you have already sedated him and he just wont wake up that is all. After leaving him in the bucket for 30 minutes dispose of his body.
also do make sure you pour quite a bit of vodka in and do wait 30 minutes because if he wakes back up he may be a toughie and you will need more clove oil to put to sleep and then pour more vodka on him. Some fish are tough and it will take a few hours to peacefully let him die without pain. Most other fish which are already half injured will just go first time
Let me know if you decide to do this or you decide to press on with leaving him and maybe he will get better who knows
I just hate to see a fish in pain!