I'm sorry to hear your cat has been diagnosed with PKD, unfortunately it is more common in Persian cats. Every drug can have some kind of side effects, but I'm surprised by him initially being put on prednisolone, I've just done a quick google that says treatment isn't normally needed. Did this years ultrasound show a reduction in his liver ones? I'm not sure I'd go down the Fortekor route without blood and urinalysis being done, it generally isn't effective until they are losing protein in the urine. Have they recommended a diet change? Sorry for all the questions!