Hi! Zak,,,,it used to be known as Glycogen Storage disease, an inborn error of metabolism.
Not my area of expertise.
From what i know it depends on what type your son has and is not easy to control, basically dealing with blood sugar levels.
No doubt that someone will come along with websites to ponder over, but they are not easy to understand.
Not the best diagnosis to have.......but there are much worse.
Zak...I have done a bit more "digging" and it seems that although this is a chronic condition (irreversible) it is NOT fatal and the future of your son depends upon the avoidance of strenuous exercise.
Sorry...that is my lot as it is an extremely uncommon disorder.