A 7-year-old boy is brought to the emergency department after falling on his outstretched hand while playing cricket. He has severe pain and swelling in the anterior elbow with the arm held in flexion. On examination, the radial pulse is palpable but weak, and capillary refill is delayed at 4 seconds. X-ray shows a displaced supracondylar fracture of the humerus with posterior angulation. What is the most appropriate immediate management?