I agree with @BigBlueHen53, the best things you can do are to keep him comfortable, minimize any chances of his bumping the sore spots and protect him from dampness and icy temps. Just like with people and pets, until that heals he's highly susceptible to more frostbite.
Once it's fully healed, you can put petroleum jelly (Vaseline) on his comb and wattles to help ward off the issue.
Years back, I had a cockerel that the flockmaster would not always allow into the coop. With a nasty cold snap coming, I knew he needed some extra protection, just-in-case, so I started putting a light covering on his "manly red parts" at night. He fought it at first, but apparently decided it felt nice. For years, he would come right up to me and lower his head for a massage!
EDIT TO ADD:
Do NOT put the goop on his comb or wattles now! That's for NEXT winter, and only to help protect his damaged parts. The best prevention is to make sure your coop has good ventilation and no added moisture. If you can keep their waterer out of the roosting area, that should help.