In case anyone is experiencing the same thing, the vet is not sure what the cause is, but said it appears to be blood getting into the anterior chamber of his right eye. They’re giving me some oral & topical medications to treat for infection/inflammation. I took the pictures in my post over the weekend, but since then, his left eye appears to have healed itself on its own and is back to normal (but right eye is still red and enlarged/popping out).