Maya also used to scratch herself and my vet gave me special oil tubes to put on her. His skin may be dry and like humans, it can be VERY itchy. From your stories, I would suggest going to another vet for a second opinion.
see below link
Dermoscent Essential 6 Spot-On Skin Care for Medium Dogs 22-44 lbs (10-20 kg), 4 Tubes - Pet Meds