A 13-year-old boy with obesity has had aching pain in his left knee for three weeks. He suddenly develops severe groin pain while rising from a chair and cannot take four steps, even with crutches. The hip is held in external rotation and flexion, and any attempt at internal rotation causes marked pain. Pelvic radiographs show widening and irregularity of the proximal femoral growth plate; there is no fracture. Which one of the following is the most appropriate management?