Few questions for you; first, is that heat pad controlled by a thermostat? If not unplug it until you get one
Second, is your snake fed live? Could be an injury from a bite or it could be part of a stuck eye cap. If it's an eye cap just leave it alone unti his next shed and it should remedy itself. Make sure your husbandry is spot on. You can also add a humid hide when you notice the signs of a shed.