Well, that's sad isn't it :(
Lots of folks take in birds, so hopefully it works out well for you.
Do you have other roosters? Others have way more experience with gamefowl than I do, but if you have other roosters roaming around, you will likely need to make sure they can't get near him.
Can you tell if that's pus or the eye?
I would try flushing with saline, press the eye and see if that's pus, if it is, then remove it.
Apply an eye ointment like Terramycin.
Do you notice anything else - mucous inside the beak, runny nostrils, coughing, sneezing, wheezing?
If you don't...