What Causes Blood And High Protein Levels In Urine Of A Child?

My 8 yo son is seeing a pediatric urologist on Thursday of this week, but I am a nervous nelly and I want to know what to expect. He has had blood in his urine for approx 2 months and last Thursday PM his protein levels were 300+. They took another urine sample Friday AM and protein levels were 30, a big drop. He has not one other symptom. He has had 2 renal US and they have both been normal. He had a KUB, normal. What could be causing the blook and protein?