It sounds like your son may have a condition called slipped capital femoral epiphysis (SCFE). In children, at the top of the femur (thighbone) there’s a growth plate. The growth plate is sandwiched between two special areas of the bone called the epiphysis and the metaphysis.
The growth plate is made of a special type of cartilage that builds bone on top of the end of the metaphysis and lengthens the bone as we grow. In this condition, the growth center of the hip (the capital femoral epiphysis) actually slips backwards on the top of the femur.
What happens to any condition that’s left untreated is called the natural history. The natural history of SCFE isn’t certain from patient to patient. It could remain stable without problems.
More likely complications and problems can occur. For example osteonecrosis can develop. This is a loss of blood supply to the bone with death of bone cells. The hip may also degenerate and develop arthritis so that the person is in great pain, loses function, and may not be able to walk.
Fortunately, the condition can be treated and the complications avoided or reduced if recognized early. Surgery now can stabilize the hip and prevent the situation from getting worse. Long-term studies show a good result with surgery done early to stabilize the hip.