It could be something as minor as a speck of something irritating the 3rd eyelid, or as serious as a scraped cornea. No way to know without a vet looking at it.
I've had the vet out a couple times for similar eye issues. Knock on wood, they were fine, but you never know.
I also have seen a handful of horses over the year with cloudy/partially blind eyes too, that I assume are from some sort of injury.
PS. A fly mask will help keep the flies out of the corners of his eye as they just love runny eyes.