Depending on how cool the weather still is, you might want to bring him into a warmer location until the circulation in his comb improves - if it's staying cold and blood isn't flowing to it isn't going to start healing. I put salve on my birds' tender combs - made from beeswax and medicinal herb oils - other have recommended vaseline, etc. Watch out for picking by the other birds. Mostly, try to prevent it from worsening and promote healing. Vitamins, healthy treats.
As far as the coop, covering windows can decrease ventilation and increase the problem. The cold they can handle better than the cold + damp. Chickens produce a lot of moisture. Keep the coop extra clean (scoop poop daily) or use deep litter bedding. Make sure they have deeper bedding in general than during the summer - 6 inches or so. You can get a decent thermometer/hygrometer to help you be aware - I have this one:
https://www.amazon.com/gp/product/B000XTJRRA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1.
My big boy didn't get his frostbite in the coop, however, but on subzero days out on pasture. Since the hens and their tiny combs were out and about, he felt that he needed to be out there protecting them and didn't take himself into consideration. He's a good rooster. He has learned his limits, however, and now chooses to stay in on very cold days and let our peacombed Ameraucana rooster handle outside duty.