Horsie Boy's Eye

This afternoon when I was about to feed Boy, I realised something was wrong with him. He kept shaking his head, I thought he was just playing, then I saw his right eye. It was badly swollen, teary marks and some creamy liquid . Not sure what the cause is, but maybe he has been quite naughty and keep rubbing his eyes to any object that he could find. Tried to wash his eyes with fresh water, but he refused. Anyway, he still has his appetite, so hopefully it is not that serious.