To tell you the truth I always leave that to the vet... so i dunno!
Stand near the rear of him (that is if its safe) and start waving your hand. Horse's eyes, being on the side of their head, allow them to see almost completely around them. If he doesn't notice your movement, there is probably something going on.
As for hearing, stand at different spots and kiss at him. See if you get his attention. I don't really know how great of a test that is though. Have your vet look at im again I would say.