hi, Thanks for posting your query in Health Care Magic, i understand your concern. The bow legs could have caused his knee pain , also because he is an athlete its possible for him to strain his legs more than other children of his age. Most probable cause for his bow legs could be arthritis, if it was present since his childhood and incase you have missed catching it, it might have been due to rickets ( condition due to deficiency of vitamin D ) . In childhood with mild bow legs it could have been corrected with exercises and physiotherapy. But in his case i think surgery is the only option. Consult an orthopedist for further help. Getting an X-ray done of his limbs will help to evaluate the degree of bowing of his legs. Depending on this the orthopedician will be able to provide with treatment options. Hope this helps. Write back to us for further assistance , wishing him good health.