Right now it sounds like only a very small part of the comb is actually frostbitten. Those tips of his comb which have turned white will eventually turn black and fall off, leaving behind a scab. Eventually the comb will completely heal up.
To prevent further frostbite, simply apply vaseline like another poster mentioned. When applying the vaseline, it also helps to massage any purple areas to encourage blood flow. Be careful not to massage the white or black ares though, as touching that too much may be painful for your bird.
Keep an eye on the rooster to be sure he doesn't develop any infection and acts normally. (Eats, drinks, and is active) Some roosters are just more prone to frost bite. Typically, after the first winter a rooster will no longer develop frostbite on his comb or wattles.