Organic cat food for feline leukemia?

I have a cat with feline leukemia. Because of this, he has to be fed a prescription food or otherwise he will develop crystals in his urine which can be dangerous. I would rather feed him cat food from a place like Whole Foods because I feel it will be more healthy food without meat by-products and hormones but when he was fed this type of food he developed crystals in his urine probably because of the ash content or the ph balance in the food. Does anyone know of a cat food which which is not commercial and will not hurt my cat that is organic? I'd be willing to mix up the food myself if I had to. So far, I have been mixing up half organic, half prescription food and he seems to be doing OK but I would like to find organic cat food that will not hurt him. Thanks.