After the initial exam and negative X-ray (we visited the first orthopedic specialist available after my son confessed he was in so much pain he didn't think he could play the next day; that admission was a HUGE warning sign to me, because son loves playing almost more than anything else and has played with more minor injuries before), a muscle strain was suspected. After a follow-up visit to the spine specialist at the practice, an MRI was ordered, and the diagnosis was a stress reaction to the L-5 pedicle, with no fracture located. Discs and everything else were normal.
It is VERY encouraging to hear your son's story!