How did your vet determine it was food based? Usually, that can only be determined through extensive testing.
We have a 4 yo German Shorthair who has had allergies most of his life. He’s on Prednisone and Benadryl twice daily. We’ve served Science Diet allergen free ZD. It didn’t help. We now serve Science Diet Large Breed Lamb & Rice formula and that seems to help. But he still has his itching and rashes. We’ve come to the conclusion that it’s likely not food based, but the only way we could perhaps find an answer is through very costly testing which in the end may not provide an answer.
prednisone will shorten his life
have you had his kidney function tested?
Try talking to a german shepherd breeder for suggestions