I had a 2 year old roo who lost his "voice". After a week and a half, I called the vet, who does treat birds, but is a long ways off, and he said start him on Teramycin powder from the feed store right away. I did put the Teramycin in his water, but after 2 weeks of it, he died anyway, being too debilitated to recover. He was eating, but too sick to heal.
I would suggest starting some kind of antibiotic immediately, quarantining him from other birds and a fast visit to a vet who treats birds, if you can.
I miss Comet still, and it's been over a year. Please take action fast, and let me know how it goes. Good luck!
BTW, that's the same thing that got me to BYC! Welcome- I now hatch my own chickens, this place is truly addicting!
