Got your PM.
First off, calm down, and don't panic... It won't help and will only make you feel worse. We all make mistakes at one time or another, it's what makes us human.
As with all injuries, clean, warm water and a low stress environment are key to recovery. 100% daily water changes will keep your parameters in check and help get rid of nasties that could take hold and cause infection. I would carefully re-acclimate him back afterwards... You don't want to cause more stress. Keeping it warm is a given. 78-80 degrees is good, too high favors bacterial growth while to low weakens the immune system. To keep stress low, keep the tank dark and quiet... A hiding spot may also be beneficial if he doesn't have one already. Stress coat is great, aquarium salt isn't... Unless you are midway through a ten day course to cure the fin rot, I wouldn't recommend using it as a preventative... It may cause more stress.
Do you have any pictures that I could assess the damage from?