If you've been to the vets over this a couple of times maybe its time to ask for a second opinion - it's your right to request this and a fresh approach to the problem may come up with a solution. Lots of dogs groan when they scratch their ears and its often a result ear mites, something stuck in the ear canal etc etc etc - can be a hundred and one things. Can understand your concern as he's a Cavi though - your vet should also take this on board - it must be very worrying for you so maybe be insistent on a second opinion? Keep us posted.