Are there any signs of injury other then the hanging head?
If not, it could be wry neck. It is a vitamin deficiency. You can try this vitamin protocol and see how it goes...
.5mls poly vi sol, without iron, daily
400 ius of vitamin E twice daily.
You can get both at your local Walmart. Will run you about $10. Try this for several days...see if the symptoms get better. I would keep him in the house in a safe, dry place until he gets better. Otherwise, the other chickens might jump on him and he would be far worse off.