Is there a reason why you are trying to keep a Cornish cross past butchering age? They are prone to leg and heart problems due to their fast growth and if you plan to keep them as "pets" you really need to limit food from the start. It is not fair to the bird to suddenly remove food and can cause more harm than good anyway.
From the little we can see of the bird in the photo, I too would suspect it has Marek's as it looks to be in classic Marek's splits posture and the age of the bird ties in with Marek's. Other birds will often attack them once they become incapacitated with it. Many people cull birds that they suspect may have Marek's as it is infectious although by the time symptoms appear, they may all have been exposed. I usually offer supportive care but with this being a Cornish Cross and having already been attacked by it's flock and injured as well as the probably Marek's, I think it would be kinder to euthanize. If it was a smaller bird with some hope of longevity and possibly recovering from an outbreak of Marek's and returning to the flock, then it might be worth persevering but with such a large bird that will have mobility problems as well as other organs no doubt under pressure and the flock having set upon it, there is little hope of any quality of life for it if it can survive this paralysis outbreak.
