Bringing him in is a good idea while it's still below freezing. He should be kept from conditions fostering further frostbite. He should not be kept in a very warm room, though, or he will lose his cold adaptation. Any temp above 50F should be avoided.
He has very large wattles. He likely dragged the wattles in the water he was drinking. In very cold weather, open water dishes or drinkers with a tray should be avoided. Nipple waterers are a safer solution, although they require a bit of effort to keep them from freezing up.
There really is no treatment for frostbite in chickens. Care needs to be taken not to rub or irritate the damaged tissue so no topical ointment is advised. Just take care he doesn't get the tissue frozen again.