Darkening on the edges of the comb can indicate that he is not getting enough oxygen. Avian Influenza can cause cyanosis and you'll often see some signs of swelling of the comb, wattles, eyes, or face. Generalized signs include depression, decreased activity, huddling, ruffled feathers, decreased feed and water consumption and occasionally greenish diarrhea. If you suspect AI in this rooster, you really should consult your State Veterinarian.
Going in a different direction...have you ever noticed him shaking his head, gaping (opening his beak as if to crow, but nothing comes out), sneezing, coughing? Those can be symptoms of gapeworm, a parasite that attaches itself to the trachea and can slowly suffocate a bird. Treatment for gapeworm is worming with fenbendazole. Antibiotics are sometimes given, as well, to treat secondary respiratory infections.
Just a couple of things for you to consider.