My initial thought was Mareks. I'm afraid the information you read is not the whole picture with Mareks by far. I have lost one quite quickly to it with huge tumours but I have one that has been lame for 4 months with it and got no worse She has learned to hold her bad leg up and hop about..... she eats and drinks and is the picture of health apart from being on one leg. She is very petite though(another Marek's sign), so there is no significant weight for her good leg to carry. Another went horribly lame 3 months ago and miraculously recovered a few days later and then had another attack a few weeks ago and has been unable to stand and needed to be propped up in a nest box as her legs stuck out and she would roll onto her side. Again bright eyed and eats well which is normal with Mareks. Thankfully she is getting a little better and has started to stand again for short spells in the past few days. It's taking quite a lot of tlc but mine are laying pullets not meat birds and the worse of the 2 has even started laying this past week despite being semi paralysed and I'm getting an egg every other day from her so far.
Having said all that, I believe meat birds like yours have problems with their legs and joints nayway because they grow so fast and it may be that your little man has dislocated hips looking at that picture. Considering that he is a meat bird and you would be slaughtering soon anyway, it might be best culling him now as I would imagine he will get soiled and pressure sores/burns from being down on the bottom like that and any meat may well not be fit to eat if he gets an infection.... Or you could make him a hammock so that he is supported up of the ground until he is up to slaughter weight.
Apologies if I have offended and he is not intended for meat as most Cornish cross are. If he is to be a pet then I would cut down on his food, make a hammock and perhaps do daily physio. You will also need to keep nails clipped as they grow long with not being able to run around and scratch.
