That is why you always start stringing the crosses at the HEAD down. The Y yoke at the bottom of the head does add strength to the hoop so it's more stable. When you begin to add cross strings pressure is pushed up or down in the head away from the crosses.
If you start from the bottom, pressure is pushed to the top of the racquet head where there is no additional support and can cause cracks at the key spots where the mains and cross strings are close. Stringing top down has the pressure moving down the head towards the throat where it's stronger so you get much less deformation, insuring racquet integrity.
Hope this clearly explains it and helps. BTW, the OP does not say what string and tension his son strung with, as poly strings at high tensions can cause a real issue with racquets and arms.
It was strung with Kirschbaum Proline 2 (17g) and tension 57. It was strung by a pro shop in Daytona Beach, FL.