Rooster suddenly ill
He definitely sounds ill with something neurological. Botulism, Mareks disease, or a head injury sound like possiblities. Botulism comes from eating the remains of dead animals or vegetation that has been without air. Paralyzed legs, then wings, and neck in that order are common. Will he eat or drink? Mareks disease is a virus that can cause paralysis of one or both legs or wings, and sometimes the neck. Severe dehydration from being kept from water or food can also cause those symptoms. Do you have other roosters? Was he vaccinated for Mareks disease? I would give him some water immediately, and if you have some electrolytes, that would be good. Here are a couple of articles that you may want to read:
Please give him some fluids and electrolytes, since if he makes it the first couple of days, he might survive. You can give him a molasses or epsom salts flush if you really think that he has eaten something bad. If he is dehydrated and doesn't have botulism, that could dehydrate him further since it will cause some diarrhea. You can use a dropper or syringe to give him liquids. An update on his condition tomorrow would be appreciated. Hopefully he will start to feel better.