A 4-year-old boy is brought to an outpatient pediatric clinic by his grandmother who reports a sudden onset of right elbow pain. She explains that while they were playing, she gently pulled him up by his outstretched arm to help him stand, and immediately he cried out and refused to move his arm. He has no history of falls or direct trauma to the elbow. On examination, the boy is visibly distressed and holds his right upper extremity in a characteristic position with the elbow slightly flexed and the forearm pronated. There is no swelling, ecchymosis, or obvious deformity. Palpation elicits tenderness over the radial head. While passive flexion and extension of the elbow are preserved without significant pain, any attempt at supination is met with resistance and causes marked discomfort. His vital signs are stable, and he appears otherwise healthy. Considering the clinical presentation, what is the most likely diagnosis and the definitive initial management strategy?