Assessing 10 year old for surgery

My 10 year old son and I have recently been diagnosed with aortic aneurysms. I am a 46 year old woman, 5'10 inches tall, 150 lbs - and my aorta measured 4.0. My son is 54" tall and 72 pounds. His aorta measured 3.1 - according to his doctor, the normal range for him would be between 1.6-2.4. They are in the process of genetic testing, but it will take 6 weeks. From what I have read on the internet, congenital patients such as myself would be considered for surgery at 4.5. When I asked the pediatric cardiologist what size my son's should be before being considered for surgery, he said he does not measure by size, but by rate of growth, and if it grew by more than 4mm a year, he would consider my son a candidate for surgery. Is this the standard way to assess for pediatric patients and if so, why? Should we seek a second opinion and if so, do you have any suggestions for doctors?
Thanks very much - Sheri