@ClaremontChicks - Do you have any photos of this fella? Often, that will help others on here to get a good idea of what they're dealing with before giving advice.
I caught my rooster with frostbite this week and while there wasn't a lot of swelling, I did end up moving him into our pantry (no heat in there, but warmer than outdoors). Is the swelling due to blistering and puss? Have you tried any remedies on him so far other than keeping him in the garage (is the garage kept at a temperature that is above freezing?)
This is an ointment that Fresh Eggs Daily recommends and I have been using it on my rooster. Just keep in mind that you want to avoid rubbing very hard on him as his wattles are already tender and damaged. Be as careful as you can. If he has a lot of blistering, you may want to hold off on this to make sure you aren't covering up an infection.
Homemade Frostbite Ointment
This all-natural ointment can be used to prevent and treat frostbite.
2 ounces beeswax
3/4 cup coconut oil
1/4 teaspoon liquid vitamin E (helps repair damaged skin)
10 drops calendula essential oil (anti-inflamatory, aids in healing wounds)
10 drops lavender essential oil (relaxant, pain reliever, antibacterial, anti-fungal)
10 drops lemon essential oil (antibacterial, antiviral)
Grate beeswax and melt with coconut oil over low heat in a glass mason jar set in a saucepan of boiling water, stirring with a wooden chopstick. Remove from heat and stir in the Vitamin E and essential oils until well mixed. Leave in the mason jar or pour into a covered container and cool. Store in a cool, dry place and use as needed.
Edited by naivetefarm - 1/5/16 at 2:15pm
In serious instances, some folks may recommend dubbing. There are plenty of threads on here about if and when this is appropriate, but if this rooster is in as serious a position as you say he is, you might want to start your research on whether or not this is an avenue you'd pursue. Some controversy around it - some folks believe it to be cruel while others believe it to prevent the bird from repeated pain with recurring frostbite... You'll have to decide for yourself if it's a useful avenue or not.
Keep an eye on him in the meantime and keep us updated. Do you know what cause his serious frostbite? Are you sure your coop is getting enough ventilation? Check for moisture buildup in there, especially in the morning right after you let everyone outdoors. What kind of waterer are you using? My rooster ended up with frostbitten wattles because I was using a heated dog bowl - each time he took a drink, he was floating his wattles around and getting them wet. Take a good look around your chickens' quarters to make sure there's nothing in their environment that may be making them more prone to frostbite.
Best wishes for his recovery!!