I know exactly the struggle you're going through. My Alexander was also determined to be allergic to chicken and it was an ordeal finding any chicken-free food a few years ago. There are many more on the market today thank goodness. Most of the chicken-free foods I found were grain free but there are some grain inclusive foods now, too. Some of my favorites are:
Nature's Variety Instinct (grain free) in the Beef/Lamb and Duck/Turkey formulas. Duck and Turkey have different protein structures so most dogs allergic to chicken can still eat these proteins.
Nature's Variety Praire (with grains) has a few different chicken free varieties.
Acana Pacifica and Acana Grasslands (both grain free) contain no chicken.
Taste of the Wild's Pacific Stream formula has no chicken. Neither does their Sierra Mountain formula. These are both grain free.
Fromm is another good brand that has some chicken free formulas.
Good luck in your food search!
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He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. -- Author Unknown