Flutd food problems

My 11 y/o cat is prone to calcium oxlates stones. He had been eating
Hill's x/d dry when he developed a food allergy. He was switched to
Hill's k/d dry but is allergic to that too (same main ingredients -
rice, corn, and chicken). The vet wants to put him on Hill's z/d dry
or (I think) on Hill's d/d Duck & Green pea dry. After he gets settled
with his food, then I can start trying to get him to eat canned food.
I realize now that he really needs to eat that instead of just dry.

Is the z/d and/or d/d ok for a cat prone to calcium oxalates? I
noticed that the target pH for these foods is 6.2 to 6.4, whereas the
x/d and k/d had a target pH of 6.6 to 6.8. It seems like the pH would
make a difference.

The Hill's x/d and k/d have a lot of corn, rice, and chicken. The z/d
has hydrolyzed chicken but no corn or rice. The d/d doesn't have any
corn, rice, or chicken. I think just about all the other food choices/
brands have corn, rice, and chicken or have reduced protein.

Does anyone have any advice or food recommendations?

The vet and I want to find a dry food that works before trying to get
the cat to eat canned food. We want to determine what is causing the
allergy and switching to canned right now will just complicate
matters.


Thanks,
Jo