A 28-year-old man with severe community-acquired pneumonia (CURB-65 score 3) is admitted to the ICU. He is started on IV ceftriaxone 2 g 12-hourly. On day 3, despite appropriate antibiotic therapy and supportive care, his clinical condition deteriorates: fever persists (39.2°C), WBC rises to 18,000/μL, and a new pleural effusion appears on chest X-ray. Sputum culture and blood cultures remain sterile. What is the most appropriate next step in management?