How is his beak and foot color? If they are pale it could be a sign of anemia. 
My Ethel got terribly sick, just like him. Could not stand, flopped over, would not eat but did drink some. I was so afraid for her that I took her to the vet, almost 600 dollars later she was diagnosed with heavy metal poisoning. You could see the buckshot in her intestines on the exray. I had to force feed her for a while and give her a shot every day for about 10 days and fed her peanut butter bread to bind the pellets and get them out of her. My hubby calls her the "Golden Goose" now, says after that $600 she should lay gold eggs for us.