Have you tried Royal Canin?
My 5 year-old Jack Russells started out life with some mild allergies. This caused some itching, and their coats weren't as lustrous as they should have been. One of them also had a sensitive stomach. So, our vet started them on Royal Canin about two years ago. They have both canned food and dry kibble. I feed them a combo of both twice daily. He has them on Duck and Potato dry kibble and Venison and potato canned. I was amazed at how their problems began to clear up. They began to have very sleek and shiny coats, and the itching stopped. The one with stomach issues stopped throwing up and was happy to eat again. Previously we had used Iams and Eukanuba.
Since then, one of them has developed an allergy to ragweed. Every August or September (until November/December)she starts itching, has red, teary eyes, and loses her appetite. She has to take oral meds and shots now. I also put a half teaspoon of Omega 3 Fish oil on their food. Even though the food is still the best thing for her, it isn't enough to combat this allergy.
Royal Canin also makes varieties for other problems and for weight issues. I buy mine through the vet so I know it's fresh, and reliable. I say that, but truthfully I don't know if it's sold in specialty stores or only vet clinics.
It's made a positive difference for my babies. I hope you find the right thing to help him. I feel terrible for him, and I know you do, too. It's difficult when we try everything and nothing seems to help. Good luck.
PS....It's made in St. Charles, MO, too!