It could be a stroke or poisoning. Many times it is hard to diagnose a chicken illness without anecropsy after death. The state vet or local poultry lab could perform one if you lose him. I hope he has some improvement with the care you are giving him.
I see no problem in treating him, and hoping that his symptoms get better. Chickens do okay in familiar surroundings with vision loss and other symptoms. The vitamins and selenium may help, and it sometimes can take weeks to see improvement. I tend to baby a sick one for a few days, and then let...
He could have a possible vitamin deficiency causing his blindness, curled toes, and wry neck. I would give him 2-3 ml of Poultry Cell vitamins orally every day, or in a treat. An alternative is to give him Poultry Vitamins in his water, and a small amount of chopped egg or tuna for extra...