wow that is simply amazing! i would suggest you put him in a quarantine tank and use stress coat +. he may never fully recover but make sure to feed him nutrient rich foods and a varied diet might also help. keep him in quarantine for about a month or until he is ready to face the others and compete for food. i would also suggest a 25% WC 2 times a week to keep away infections, and keep the temp steady because right now he is probably very sussecptible to desiese and needs the best water possible. and one more thing, give him lots of love and attention.

good luck.
EDIT: seeing that you dont have a quarantine tank i would use a tank divider so he isnt picked on by the others.