Pug has allergies and senstive stomach, Help choosing a food please!?

After adopting my pug from a shelter I noticed he chased his tail and chewed it raw. I immediatley took him to the vet, he had yeast in his wrinkles, a ear infection and dry flaky skin. They prescribed a ton of pills and ointment for the ears and said it was a food allergie. He was eating Science Diet, the natures best lamb and brown rice one. So the vet switched him to Eukanuba. He did not do well on that food at all, he seemed worse. I purchased taste of the wild pacific stream, he actually would dump his bowl over and refused to eat it for like 3 days, I even did the mix it with the old food and gradually adjust him. He would just not even eat a bite of it. So I go to petco and purchase Dick Van pattens duck and potatoe. That food gave him clear the room gas and horrible diarrea. Back to the vet and they said to refrain from food for 24 hours and let his stomach rest, so since petco has there 100 percent return policy I took back the duck food and got the fish and sweet potatoe one. He has been eating it a little over a month, but his hair looks horrible, in that area he chewed has yet to grow back and he still has very bad gas. He doesnt have that shiney coat look and its still dry and flaky and even with the pills he still itches. So please if any one can recommend a food for a allergy ridden dog that helps I am all for it. I was thinking of trying Wellness food next or blue buffalo.
thanks so much!