It could be a seasonal allergy where the itching has got out of control. Once the skin is irritated and open from the scatching other infection has an entry point. With my boy, also a Lab, we have decided we have seasonal allergies instead of food allergies because it cleared up in the cool weather on the same foods. I jumped on the itching, accompanied by the most awful smell, very quickly and keeping the itch controlled removed the smell and did not allow infection to develop.
It sounds like you are past this point, but in case you want another option to try, I'm dosing with Vanactyl P every second day and Reactine twice a day. I'm actually tapering off the Vanactyl P right now. If you can get your boy cleared up over winter this might be a way to start out next spring when/if the itching starts up again. Hope our experience helps.