rawhide bone allergy

Where is the rawhide from? Some is processed in China...a scary thought. The way rawhide is dried may contain chemicals. But I doubt thats the reason for itchy's.
I think he may be scratching due to the dry air of Winter. Do you run a humidifier in the house if you run a furnace? Most dogs are getting the winter coat in right now so are on the itchy side while it comes in/add in the dry temps and it seems all are scratching. Are you supplementing with oils, and feeding a good kibble?

I would keep him on the fish based for awhile longer. Allergies take time to get out of the system if it was food based.
Instead of Nutro(I don't think it is a good food) Natural Balance limited ingredient would be a good choice or a fish based(which you are already feeding) if you think there is a chicken allergy. Make sure you look at the ingredient list to be sure no chicken is included.
Benedryl takes the edge off as well. Dosage is 1mg for every # the dog weighs.