1 week old baby chicken......Help!?

Hi everyone! So two days ago my sister bought 6 baby chickens at the local pet store. The chickens are 1 week old Silkies. The baby chickens were very healthy at first......they had the proper lighting, bedding, and feed. Last night the smallest chicken seemed fine.....he was sprawled out and sleeping under the light when we said good night to him (not sprawled out as in sick). Early this morning the baby chicken woke up and started circling uncontrollably and crying. My sister took him out of the cage and held him until he calmed down. All morning he has been sleeping but if you pick him up and set him on flat ground he will lay his head backwards and starts spinning in a circle to the right. He is starting to refuse water and food, and prefers to sleep while being held. His body seems normal so I don't think there's a problem with his legs or anything. I really don't want him to die, and we're doing everything we can think of to help him. I am starting to think that something might be wrong in his brain:[ Has anyone had a baby chicken that had a similar illness? If so what did you do to help the chicken? Did the chicken survive? Any answers would be greatly appreciated!! Please no mean responds!