A 7-year-old boy from Delhi presents with acute onset fever, severe left knee pain, and inability to bear weight for 2 days. Examination reveals a swollen, warm knee with restricted motion. ESR is 68 mm/h, CRP is 12 mg/dL, and blood culture is positive for Staphylococcus aureus (MSSA). X-ray shows soft tissue swelling; no bony changes yet. What is the most appropriate next step in management?