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Poor little Fergus sounds so uncomfortable!
I think there are probably at least a couple different things contributing to his itch.
I agree there is likely an underlying allergy, which can be an environmental allergy or a food allergy. We have not yet ruled out food as a cause, and I'll explain why.
First, most dogs are not allergic to grains themselves. They are allergic to a protein source. Although grains do have some proteins in them, the meat is often the culprit. For example, chicken is one of the most common food allergies in pets. Rather than switching him to a grain free food, it would be best to try him on a hypoallergenic food such as Hills Z/D, Purina HA, or Royal Canin HP.
Then, it can take 12-16 weeks before we see the full effect of the food - 3 weeks is not nearly long enough for a trial. Also, there can be no other human food or treats (unless they are specifically hypoallergenic treats) during this time.
Another reason he may still be itching is that dogs with allergies as their primary problem frequently develop secondary bacterial or yeast infections from inflammation, itching and chewing at themselves. These secondary infections are ALSO itchy, so we must control BOTH the secondary infections as well as the primary allergy to resolve the itching.
I haven't finished yet. Itchy dogs are complex. Do you want to hear the rest?