What I did for my boy once I new what is was, was to lower the protien in his diet, he still died from renal failure, but he out lived his full litter mate sister by just under a year. To much PHOSPHORUS is also a bad thing for them and it can build up in them so a food with a lower PHOSPHORUS count would be good too, but CALCIUM and PHOSPHORUS have to be in some kind of balance to work, if my memory is working it is like 2:1 Calcium to Phosphorus.
As this article says check with your vet and there are some medications available to help, but at the time I was working with my 15+ old Persians we decided diet would be the biggest help for them as they were on a HIGH protien food.