Many causes. He should be evaluated for proper diagnosis and treatment options.
Where is the pain. The location and type of pain is important. Is there joint swelling? Are the boys growing fast - could be growing paints. Do they exercise or are they video game couch potatoes? This could be deconditioning. Stated as long term - how long? Is the course changing for the best? For the worst? Where is the joint pain located? Which joint (s)? Which side of the joint? We need help guidance.