Have you had him long? Have you ever had any chickens with coccidiosis? He may have been exposed to cocci if you have recently brought him home or maybe another chicken from another farm. There are 9 strains of coccidia that chickens get, and they usually have built up some immunity by this age, there may be a strain that he is not immune to. It is treated with Corid (amprollium) for 5-7 days, and treatment won't hurt him. Another consideration could be Mareks disease from the description of loss of balance. Was he vaccinated as a day old chick?