check his eyes for wetness or swelling.
does he have any mucus from the eyes or beak?
does he have any gurgling, wheezing or other breath sounds?
check his beak and make sure the nostrils(nares) are clear.
is he eating and drinking as usual?
what all do you feed?
does he act sick?
describe his droppings..color and consistency.
roos hide their illness till they are very sick.
he might need an  antibiotic, such as Tylan 50 injectible ..or erythromycin..(sometimes called Ery-mycin, or Gallimycin) or LS-50..
sometimes Areomycin or Terramycin can help if caught early.
good food, extra protein and vitamins will help..
keep him warm and out of draft  and wet.
if you don't have poultry vitamins, you can use Poly-vi-sol liquid baby vitamins, Enfamil brand if possible..no iron. give 2-3 drops on beak once a day for a week.
what bedding are you using in your coop?
is it a closed coop or shelter?
has it been cleaned recently?