Either an old eye injury or retained eye caps. Pretty common with wild snakes and should in theory heal over given time. Best to just release him and if he is destined to become food for something, let him. Nature must take its course, especially since corn snakes are by no means endangered. Make sure to keep him away from your snake as well, don’t want anything to be released into the wild from captive snakes. You also don’t want to possibly give your snake something.