I don't know what it could be but to make it easier for others to answer:
Does it feel hard or soft?
His eye looks odd to me, is it always that cloudy looking?
Take a look in his beak, do you see anything down there that looks wrong? Any yellow or white lumps? Is there a smell from his beak/nose area?
How is his appetite? Is he drinking? Any symptoms in the rest of the flock? And what do they eat?
Answers will help those who follow to better advise, hopefully someone will be along shortly. Best of luck with him!