Eye problems are nothing to mess around with. Look at the eye and see if there's anything obvious on the surface (a scratch, a piece of hay) that might be irritating it. Is it tender? Is he blinking more often than usual, or keeping it partially closed? Is it leaking fluid, indicating an infection?
Call the vet out and have him examine the horse within a few days, before the problem gets worse. Depending on what it looks like, she'll probably use flourescent strips to try and find any scratches.