This could be a few different things, my top thoughts are vitamin deficiency, head injury, or disease.
Treating for a vitamin deficiency is a good place to start no matter what.  Give him a vitamin E pill and part of a b complex pill daily and see if there is an improvement over the next few days.  There is a lot of info on BYC regarding vitamin deficiencies, dosages etc to treat with.
A head injury would also be possible if he hit his head or got pecked hard by another chicken.  There may be some swelling pushing on a nerve or causing some dizziness.  If it's an injury, supportive care is best which would include vitamins and rest.
Disease would be the toughest to diagnose and treat.  There are many viruses that could cause inflammation or tumors that could put pressure on nerves causing these symptoms. 
Also, check his ears, maybe he has an ear infection or something bothering an ear causing equilibrium issues.