We went to the pediatrician for a follow up today, and she said that Alex most certainly has a guardian angel. She said that since he seems to be acting normal and he is moving/functioning symmetrically, we don't need to watch him any closer than we would watch his brother. Kids are amazing.
Of course, we will continue to be extra careful with both of our little monkeys who just LOVE to climb and still have no fear. We will also remember that we did get lucky and luck doesn't last forever. As an added precaution, we put up some higher fencing on the balcony where Alex fell, and, so far, they haven't shown any interest in checking it out. Thanks to everyone for all your concern, good wishes, and prayers. Here's a picture of the "new" fence:
"Luck never gives; it only lends." ~Swedish Proverb