If you are in a cold climate, you would not be seeing fowl pox, a virus spread by mosquitoes in hot weather. I would agree that you might be seeing some frostbite. Do not put any creams or ointments on it, since they can prevent it from healing and massage or rubbing can damage frostbitten skin. You could put iodine or Vetericyn on it if you bring him inside for it to dry.
As @Granny Hatchet said, pictures of feet would be good to see if there is frostbite or other problems.