A 7-year-old boy presents with fever, left eye swelling, and proptosis for 2 days. On examination, the left eye shows chemosis, lid edema, and ophthalmoplegia (restricted upward and medial gaze). Fundoscopy reveals disc edema. CT orbit shows inflammatory changes in the left ethmoid sinus with orbital fat stranding. Blood culture is pending. Which organism is most likely responsible, and what is the immediate next step in management?