A couple of questions:
What does his poo look like - any signs of blood or intestinal lining?
How long have you had him and how long has he been like this?
My first thought would be coccidiosis - it is more common in chicks and young chickens than in older birds. In the USA you can get Corid which is an effective treatment for cocci, and from what I understand it is not too hard on a bird's system. I would therefore be inclined to treat the whole flock, as cocci can often infect birds without too many external signs and symptoms until it is too late - chances are if one bird has it then the rest will too. (I know from personal experience - I failed to spot the symptoms in time and lost one of my young chicks before I realised what it was.)