He can go back as long as they do not peck his comb. I would do it when you can observe and be with them for awhile. I would not keep him in a heated spot if he is going back out into the cold weather coop now or eventually.
Yes that is frostbite. I usually don’t recommend putting anything on it that could freeze and make it worse. No ointment, creams,,etc. It will heal with rounded off comb and wattles in several weeks to a couple of months.