Anyone here experienced with kidney problems in young cats?

Not exactly what is going on with the little man, but it is not common for to cause any renal (kidney) disease. What exactly do you mean he has had a lot of blood? Do you mean blood work? Also, how low is his urine specific gravity? If he is on IV fluids, it will be lower than normal. My guess is acute renal failure (usually toxin or infection in young animals) versus congenital disease.