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 dont 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