As you have now discovered, sports athletes are not immune to low back pain. Bony defects such as spondylolysis and spondylolisthesis present from birth or as a result of stress fractures from overuse can be a common cause of painful lumbar instability. In the case of spondylolysis, the supporting bony column (called the pars interarticularis) fractures. As you pointed out, if the fracture displaces (separates) and the vertebral body shifts forward, the condition is referred to as spondylolisthesis.
Most athletes would prefer a nonoperative approach to treatment — but preferably one that gets them back on their feet and returns them to full participation quickly. In an effort to identify the most helpful conservative (nonoperative) care for these patients, a group of physical therapists conducted a recent systematic review and reported their findings.
They conducted a computer-assisted search of articles published in English over a span of 46 years (from 1966 to 2012). After gathering all the acceptable articles and compiling all the information, the authors found that many of the studies collected (and reported on) different things. There wasn’t enough consistency across studies to make comparisons with meaning. The authors report “limited investigation” and “lack of homogeneity” as the two main reasons there was no consensus on the role of conservative care or on outcomes of nonoperative care versus surgery for this condition.
There was one other major stumbling block in studying the effects of exercise: poor patient compliance. In other words, the patients didn’t do the exercises as prescribed (or didn’t do them at all)! With the limited evidence available, the best that can be said is that surgery (over conservative care) seems to be most effective for higher grades of vertebral slippage. And exercise to strengthen the core muscles (abdominals and trunk stabilizers) decreases pain and improves function.
No evidence but clear consensus (based on expert opinion or case studies suggest) suggested that bracing works better for healing the fracture when compared with restricted activity for children and teens with spondylolysis (fracture without separation). In adults, lumbar exercises helped some people recover and return to work. But there was inconsistency in the most effective type of lumbar exercises. For example, some people responded to extension exercises better than flexion exercises and vice versa.
The lack of consensus or evidence-based agreement on the best way to treat these patients must be addressed in future high-quality research. In the meantime, your question goes unanswered as there is no known treatment protocol that yields the best results. The physician and physical therapist who are working with your son will be the best sources of information. Their advice and recommendations will be based on your son’s age, severity of the spondylolisthesis, and goals for return to activity and sports participation.