Fandango, the swelling on the chest is the crop, sometimes full sometimes empty.
 
I don't know but that buff looks sick , not like Marek's.  With Marek's , aside from their symptom, they act and eat normal.  At the very very end they'll be sleepy and pass bright green poop. 
 
I would put them on antibiotics, like Tylan.  Have you thought of Coccidiosis?  I had both at one time a few years ago.  They all got Tylan and Corid in their water.  Your one can't walk but is this from illness?  Or is it paralysis.  Hard to tell, but if he looks sick, treat him.  It may not be Marek's.