Do you know what the drainage on his leg is? Is it just on one leg? Do you think it is a symptom of his frostbite, or from the wrapping, or from pecking? That should be sprayed with your Vetericyn or wound spray, or cleaned up with soap and water. Is there any odor?
The benadryl spray seems to be for pain and itching, but doesn’t have any disinfectant in it from what I am reading. Vetericyn or Theracyn is good to use, or some betadine. First Aid Antiseptic (povidone iodine 10%) can be found at WalMart and Drug Stores.
I would try to get him out of the crate walking. If his leg stinks or is draining, you might want to clean it well or even soak him in some Epsom salts or soapy water with Dawn or Palmolive. Even salt water, 2 tsp of table salt per gallon of warm water dissolved and soak him for 15-20 minutes. Then spray with Vetricyn or put neosporin ointment on the wound. Is his crop leaking any more? Eating and pooping okay?
What is the normal color of his legs? The peeling and black skin is above his knee? Have you looked at any pictures of frostbite in chickens? I would Google some and look at toes, legs, and feet. Usually with frostbite, it gets the toes or feet, not the legs. Could the vet wrap have been too tight around his legs? Vet wrap will move around, and can cut off circulation sometimes. Is he moving his legs and feet okay?