Is he vaccinated for Marek's? Mareks causes partial paralysis and can cause them to be unbalanced. It cases tumor formation in muscle tissue, nervous tissue and visceral organs. It can come up seemingly out of nowhere. Usually, the first signs are stumbling or limping (sometimes falling back on their butts) or a droopy wing. More advanced signs can be clouded eyes (from tumor formation in the iris), labored breathing and inability to walk or stand. There is no treatment or cure for the disease. The only defense is vaccination as day old chicks which does not stop them from contracting the virus but prevents formation of tumors (which are what end up killing the birds). I am hoping that you will say he is vaccinated..... but if not and you lose him, I would send him into your state vet for a necropsy. Depending on the state you might get it done for free. Good luck!!