I assume that your son suffered a fracture of the joint surfaces of the upper two vertebra (C1-2) and the healing of the bone was somewhat displaced (off-set). This makes the joint surfaces incongruent (creating a step-off). When the two vertebra move on each other, this uneven joint surface “catches” and creates a painful “pop”.
A CT scan can reveal this. Occasionally an MRI can also reveal this finding.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.