My Son, 23 Years Old, Was Diagnosed With Medullary Cystic

My son, 23 years old, was diagnosed with medullary cystic kidney disease a few years ago. he has beent treated for Hypertension since he was 15 while going through several back fusions surgeries which was complicated by MRSA. His ultrasound from last year showed his left kidney at 11.4 cm and this year at 9.6 cm. The right kidney was 10.4 last year and is 10.3 this year. Would a decrease in almost 2 cm be considered extreme? at what point will this stop to function properly and what would be expected for the future? I do not have his lab results yet however a urinalysis 3 months ago was normal. There is a positive family history for Polycystic Kidney disease and another son has this form and no one is clear why this one has Medullary cystic kidney disease confirmed by CT.