1。fleas
2. allergy( grasses, pollens, etc.), contact (allergic to a shampoo, food dish) or systemic (food allergy).
3. Skin infections
Next to fleas, allergies are the most common cause of itching and scratching.
u should stop using the shampoo and anything that might be the source. Bathe your dog *regularly* using COOL water, not warm or hot water, which will make the itchy worse. In the water add some baking soda to increase the soothing effect.After that, PAT your dog with a towel. DON'T RUB his skin.But this is only a temporary solution.
Also you could try giving him some marine oil, as fatty acid supplyment( can get in any health food store).It might take couple of weeks to see the effect. But it's a long term treatment. Good for him anyway~
The best is to consult your vet!!! If it's allergy, he will give you medicine which will bring huge improvement in short.