Hi D.,
I don't know much about the different types of heart murmurs, but my husband was born with a heart murmur and he's as fine as fine can be heart-wise. He's a weight lifter, runner, and very physically active (and physically fit). He's even a colon cancer survivor. I'll try to find out what 'type' of heart murmur he was born with, but in the meantime, I hope it helps to know that there's at least one type that does turn out to be 'no worry'. I wish your family the best and will send up a prayer for your son's health.
Btw, Congratulations on your new parenthood. I have a girlfriend on the East Coast who had twins via a surrogate several years ago. Medical science is amazing (with heart care, too). Take care!
M. B