Rooster with one squinty eye

[frontyard chickens]

He was lethargic on Thursday, took him to the vet. They said he had a little bit of mucus in the back of his throat so they sent us home with an oral antibiotic. Quarantined him right away. It's now 4 days later and I notice he's not always opening his right eye but it doesn't appear swollen. Attached are photos of each eye.
Additional info: 7 month old black Australorp. 7lbs. None of our other chickens are exhibiting any of these symptoms. He has been eating and drinking normally. He is 1 of 2 roosters in the flock, and is the dominant rooster.

 

[rebrascora]

Fantastic photos!
How old is he? Looking at that eye and him closing the other one, I'm thinking possibly Marek's. It can cause eye discolouration and an inability to keep an eyelid open as well as a multitude of other more serious symptoms. It particularly affects adolescent birds but if they survive an initial attack, they can have outbreaks later in life and this time of year with the days getting shorter and the stress of moulting, it can cause older birds to present with symptoms too.
Do you know if he was vaccinated for Marek's?
How is his sight and depth perception? Is he able to locate food and water ok?
Can he move normally? Any stumbling or walking backwards or odd gait?