An 8-year-old boy presents to the emergency department 2 hours after a fall from a height of 6 feet onto an outstretched hand. On examination, he has severe pain and swelling in the elbow region with the forearm held in flexion. Neurovascular examination reveals absent radial pulse, normal sensation in the hand, and normal motor function. X-ray shows a displaced supracondylar fracture of the humerus with posterior angulation. What is the most appropriate immediate management?