sounds like hes banged it on an ornament or something rough..check the tank and remove anything that had sharp corners. other fish can also damage it so sit and watch him.
dont think theres much you can do just wait and see if it heals
My moor used to get it often - almost constantly. The blood goes away on its own, however i think it is due to cramped space (or mine seemed to be). As soon as she was moved from the 5 gallon to the 30 gallon any blood in the eye has stopped and not returned. If you already have a big tank then i dont know