I fully understand the panic you feel, and if it's laced with guilt, it really makes it feel awful watching this process take place. Have you given your rooster some baby aspirin as I suggested? It really can help with the pain. One 81mg aspirin in the morning and one at night can help him sleep comfortably.
I've given aspirin to my rooster when he had severe frostbite. And another hen benefited from aspirin as she was going through losing parts of her foot from infection. It looked just like your rooster's feet, only not quite as bad.
Chickens get frostbite every winter, and I bet there are thousands from this hideous deep freeze this winter in the east US. Rarely is infection an issue. The chickens' own immune system takes care of that by walling off the tissue that was destroyed. No antibiotics are necessary, nor will an antibiotic miraculously restore tissue that has been killed by freezing.
All you can do is what you are doing - keep the rooster as comfortable as possible, cushioning his feet with plenty of soft nesting material to reduce the pain as he walks on his feet, give plenty of fluids, and a supplement like Poultry Nutridrench can help boost his immune system so his body can get through this ordeal.