If he has been on the same diet and has been itching regularly, have you considered doing a food trial to check for allergies? Basically you'd pick a single protein, limited ingredient diet or (not cheap but tested) a veterinary hypoallergenic dog food and feed it and only it for 6-8 weeks. No treats, no supplements.
When he is groomed, what products do they use? Does itching increase after grooming?
When he is groomed, what products do they use? Does itching increase after grooming?