Bilateral Simple Cysts in a 7-yr Old

My 7yr old son has 3 simple cysts in left kidney and 1 simple cyst in the right.  They vary in size between 1cm and 2.9cm. He had cysts ever since he was born.  The only reason we discovered this was because during my pregnancy with him, my OB noticed one of his kidneys was dilated. He said this is a harmless and common condition, and should be monited via ultrasound upon birth.  One day after his birth they did the ultra sound and discovered one cyst.  2 months later they discovered more small cysts on both kidneys.  1 year u/s showed only one cyst on one side, and 2 on the other.  His 2 year u/s showed 2 cysts on each side.  His 4 yr u/s showed 2 cysts on left kidney and no cysts on the right.  And the latest shows 3 in left and 1 in right.  I am very confused.  How do these cysts come and go?  The doctor's concern is that the cysts have gradually increased in size, and they are saying there is a posibility that he has a polycystic kidney decease.  We don't have any family history at all.  I have spoken to 2 doctors we have in our family, and none of them are concerned.  They say as long as these are simple cysts and are not causing any symptoms, then there is no need to worry.  But I am worried of course.  My son has no symptoms, his urine and blood is normal.  Any feedback would be appreciated.