Have you had his ears checked by a vet? It almost sounds to me as if his ears hurt. Have you tried to touch his ears yourself and see if you get any response? Your comment about him snapping when the groomer started washing his ears leads me to think it might be related to paid. So many times acts of aggression are due to pain. Of course, getting close to his face may have made him feel more vulnerable and he lashed out in fear.

There are also some spray on shampoo products you can get at the pet store that are pretty effective. We use them at the shelter because we don't have very good washing facilities.