A 7-year-old boy from Delhi presents with a 3-day history of fever, left eye swelling, and inability to open the eye. On examination, he has proptosis, chemosis, ophthalmoplegia (restricted upward gaze), and mild vision impairment. Fundoscopy shows disc edema. His temperature is 38.5°C. A contrast-enhanced CT orbit shows inflammatory infiltration of the orbital fat with obliteration of fat planes. Blood culture is pending. Which of the following is the most appropriate next step in management?