Roosters can get some sort of virus that causes them to lose their voice, we have had that happen a few times over the past years. They can get quite ill so you really need to start them on antibiotics, Tylan or at the very least Duramycin, 4 teaspoons per gallon, for ten days to two weeks. Isolate him so you can treat him, though if it's what ours have had, it doesn't seem to be very contagious, I have never seen it spread to the others, we've only had isolated cases. It is important to start aggressive treatment right away though b/c once they start showing symptoms like that, they're already pretty ill. Give him whatever he likes to eat; keeping them eating is important; ours like cottage cheese, apple sauce, scrambled or chopped hard boiled eggs, cooked oatmeal, when they are sick. As in any time one is sick, start the others on vitamins and disinfect everything you can with Oxine or bleach, and watch the rest of the flock for symptoms.