Could it have some kind of poison?

Sounds serious. But need a lot more info. is he old ??

I'm gonna toss out some ideas...

It could he a respiratory illness in his head causing a loss of balance. Does he have any other symptoms, such as wheezing when he breathes or nasal/eye watery discharge? Hold his chest up to your ear and listen--hear any gurgling? Is he eating, drinking and carrying on as usual? Any strange poops?

It could be Botulism. The result can be a condition known as Crookneck/Limberneck. Check your feed--if it smells musty or moldy, throw it out and get fresh. Warm, wet conditions can cause problems for chickens since mold spores can be rampant in such conditions. You can treat for the balance disorder using Selenium and Vitamin E, but that's only treating a symptom--you need to remove any source of poison and treat the bird for poisoning if you suspect it. Check your yard for toxic plants and make sure no nasty neighbors have been poisoning your birds.

Does he have a head injury? I good strike to the head can cause swelling of the brain, and the resultant fluid build-up can cause the same Crookneck/Limberneck condition.

Let us know more details and maybe we can help narrow it down.

Velvet ~:>