Hey, Welcome to Fishlore.
He may have fungus or a bacterial infection from the damage. If you can post a pic, we could be more certain but I would recommend changing 25% of his tank water daily to help keep it extra clean and treat all the water with Prime and Fish Protector. If you can't find Fish Protector, it can be ordered online (
www.thatpetplace.com) and you could use Stress Coat for now.
If you can find liquid vitamins like Vita-Chem and a garlic supplement like Garlic Guard, both would be good to add daily (1-2 drops) after a water change. Vita-Chem promotes faster healing and garlic guard is to help with preventing infection.
Triple Sulfa and Rid-Fungus would be good to pick up if you don't have them in stock. You might need to treat him with those if there is infection or fungus.