Puffed up, not engaging with other members of the flock.
I’ve treated my entire flockwith Corid, including Satan.
His poo is green, more watery than anything.
Do you have photos of the poop?I’ve been reading up on the type of Mareks you described, and I do think he has went blind.
Will he react to your hand/fingers being near the eyes, if you try to touch the eye will he close it?
You treated with Corid-what dosage?
Is his crop emptying overnight?
A good clear photo of the eye(s) would help - do you have someone that can hold him while a picture is taken?
It's possible that he may be suffering from Marek's, sometimes it can show up in older birds. If you can take some stool samples to your vet that would be good too.
Check him for lice/mites and any injuries, feel the crop and see that it's emptying completely overnight. Do what you can to keep him hydrated and eating.
Keep us posted.