You might consider putting some Vet RX in their water for a while - it helps in preventing certain problems. And some Avia Charge 2000 in their water, also. It provides a multitude of nutrients and dissuades them from pecking at each other.
You must, without haste, remove the picked rooster from the flock and nurse him back to health before re-entering him into the coop with those who are now comfortable with picking at him. You will surely see a more developed and difficult to contain picking problem if you don't. Do it now.