Many symptoms attributed to mitral valve prolapse in the past have been found to be due to something else. There is no absolute recommendation as to how often to do echocardiograms. This should be decided by you and your doctor based upon your history and physical examination. The heart murmur itself is simply a sound that blood flow makes in the heart. Symptoms would depend on the cause of the murmur.
Some murmurs are normal innocent murmurs in normal hearts.
Others indicate a narrowed valve, a leaky valve or a hole in the heart. He is 7 now. He is doing great. I am pleased to learn that your child is doing well. The Fontan procedure has been a great operation for children with some complex heart problems.