You could use baking soda to neutralize your acidic water but at this point you will be doing frequent water changes to keep the water pristine so he can heal. This also means you'd have to dose your tank every water change. You should do a 25-35% water change daily while he's trying to heal.
A safe dosage of baking soda to start out with is 1 teaspoon per 5 gallons of water. After 24 hours test your pH in the tank. Try to keep it above 7.0.
Another option is using crushed coral. If you can't find it locally you should be able to find it online. This will also raise pH and keep it fairly consistent which is important.
Add some aquarium salt to your tank. Dose at 1 teaspoon per 5 gallons of water. Continue adding with the water you change out.
you should treat him with a bacterial medication and maybe even a parasite medication. Sometimes these two go hand in hand with fin rot.