Does he have any other symptoms? The atrophy in his face sounds like paralysis of the nerve, maybe due to trauma that may have caused the eye to swell like it has. Normally nerve paralysis like that would cause other issues in the face though, like a drooping lip or ear on that side, or as tinyliny suggested, problems eating/drinking.
Is he fed hay, and if so, is it particularly seedy? My boyfriend inputted saying a horse on his mum's old yard had a grass seed work it's way into it's tear duct, causing similar symptoms to your boy's.
Whatever it is, I really hope the vet gets to him soon, eye problems are really not to be messed with.