wow, that looks bad. Could be moonblindness (uveitis) in which case you would want to get it sorted asap as it will eventually cause blindness. Symptoms include: clear discharge from the eye, discoulouration and clouding, redness around the eye.
I don't know much about moonblindness or its causes but I know its one of the most serious eye problems a horse can have, its common enough,and its very painful to the horse.
It also stays coming back and needs agressive treatment. I would get him straight to the vet. Good luck, hope he's okay