How old is Stanley? And, how suddenly did this start (was he fine one day, or has this slowly progressed)? I can think of three things that could be wrong: Marek's Disease, Botulism poisoning, and an ear infection. Marek's is most common in young birds around 6-10 months old. It progresses slowly and slowly causes paralysis. Botulism comes on quickly and causes droopiness, inability to hold up head/wings, and trouble walking. Botulism occurs when a bird eats something dead or rotting or otherwise comes into contact with it. Lastly, an ear infection can cause incoordination. But, you would probably notice swelling around the head and ear area if it was an ear infection.
If you haven't already, I'd isolate him from the other birds. If it's Marek's, there's really nothing yo u can do and it may be best to cull him. If you suspect Botulism, act quickly, as it usually kills in 24-48 hours (if your chicken has had this condition for that time or longer, it is unlikely to be Botulism). Botulism can be treated with a molasses flush, which removes the poisons from the bird's system. Mix four tablespoons of molasses in one quart of water. Give him this mixture for no more than twenty-four hours. Make sure that he drinks plenty of it. If it's an ear infection, you'll need to treat with antibiotics. However, i think that this is the least likely option for now.
Sometimes, a blow on the head can cause neurological problems as well. Is it possible that he jumped down from a roost and hit his head on something?
