Since you don't know just exactly what Rusty is reacting to, it's impossible to say that food X or food Y will fix the situation.
You need to take a look at everything you've been feeding since the scratching started, then compare that to a number of products that are formulated differently.
Unfortunately, not only major ingredients like the sources of meat and carbs can be the cause, but also lesser ingredients, right down to certain vitamin or mineral supplements.
Due to that you could be extremely lucky and find a food that works out right on the first switch, in the worst case you could test numerous brands and nothing will work. In that case, the only way to truly find out is an elimination diet made from fresh ingredients.