The only way you're going to stop the itching for good it to identify and eliminate the allergen. Unfortunately, it sounds like that was attempted and unsuccessful.
One of the big causes of allergies is food allergy. The allergen in food is the protein sourse. Given that your dog is on a chicken protein, I would guess that this may be causing the allergies. Chicken, beef and lamb are common allergens in dogs. What I would do is try a hypoallergenic diet
. The best one available is Hill's Z/D. They make this diet by blending down the protein so that it's so small the immune system cannot recognize it. It's a fairly expensive food but it works in many of these cases. The hard part is that you have to only feed Z/D and no other protein sources (no protein treats
, chews, table scraps, ect). You also have to feed it for 4-6 months before you see significant improvement so it's not a quick fix.
As for Benadryl, you can definitely use that in the transition period at 1 mg/lb every 6-8 hours as needed for itching.
I hope this helps, let me know if you have any other questions.