Iams food allergy itchy westie puppy?

I have a 13 week old west highland white terrier puppy and I have been feeding him on Beta Puppy (same food as breeder) but when I took him to the vets for his routine vaccinations the vet said that the hair along his back looked rather dry and I would need to keep on top of it to prevent problems, he recommended changing food to a better quality food and giving him Evening Primrose Oil supplements, we have gradually introduced his new food Iams puppy food, and he has now been on it for 5 days, however for the last two days he has started itching and biting at his fur and skin and he also has bad flatulence, upon reading further I have heard that Iams is not as good quality as I thought it was and is full of fillers. Can anyone tell me if they have experienced this or have any information on this, we have decided to take him off of it and put him on Orijen Puppy food after lots of research but if you have any information I would be interested to hear. I know that westies can suffer skin conditions but I am suspicious to how quickly it came on, also he has had no environmental changes, or near pesticides etc as he is not allowed out of my back garden yet. A lot of people recommend feeding a "human diet" but I am against this as I think it would require a lot of Research to ensure it was reaching there dietary needs.