I've had two persians with PKD - but they were both around 14 yrs old when they succumbed to renal failure. I adopted them as adult cats, Suzie was 10 when I adopted her and Jasper was 12. Not so much was known about PKD when they were bred.

I've found that persians do better on a diet with no grains/cereals (as do most cats really!) - but persians in particular are more intolerant of any carbs. So, although Applaws is a pretty good food, as you mentioned, it does contain a small percentage of rice, JWB is high rice content and Gourmet souffle contains milk and dairy, and plant products (usually soya). Perhaps the combination of grains in the Applaws and JWB and the milk products in the Gourmet are the culprits? It would be worth trying him on high meat content wet foods with no grains/cereals/veggies/milk or dairy, or even try a raw diet, (obviously make any changes to diet slowly)

My persians do well on a mainly raw diet with some wet foods like Hi Life Real Meat pouches, Forthglade, sometimes Bozita and very occasionally Applaws.