You seem to be pretty attuned to this fellow in spite of him being a "loner". Do you think he will allow you to handle him?
The treatment for the swollen eye is the same whether it's caused by an injury or respiratory disease. You need to flush the eye with saline, then put terramycin eye ointment in the eye twice a day until it clears up. (Or use original Neosporin.)
If that doesn't effect improvement, then you may need an oral antibiotic such as Tylan or penicillin. He can have a baby aspirin directly onto his tongue twice a day for pain.