Are there any other symptoms? Drooping wings, half closed eyes, loosing feathers, making funny sounds when breathing? Give us all the information you can think of.
Is it possible he could have gotten into poison? Rat poison or eaten something bad for him.
Did you dust him for mites and lice? Did you check him all over his body? He could be injured and with all those feathers it's hard to see a problem. Go over him with a fine toothed comb.
If all else fails I would worm him with Ivermectin. Use the kind for cattle. It's called 'pour on' because you pour it on cattle but you put drops of the solution on chickens. For a 6 month rooster I would give him 6 drops. You place the drops on the skin, on the back between the shoulders. Make sure it gets on the skin and not on the feathers. If you get it on the feathers it will not work.
If he has a gape worm growing in his throat this will kill the worm and he should start to get better but without more information that's all I can suggest.