A 52-year-old man is 4 hours post-op following open cholecystectomy. He is on intravenous PCA with fentanyl (20 mcg bolus, 8-minute lockout). He reports severe pain (8/10) and has pressed the demand button 15 times in 20 minutes, but the pump has delivered only 3 boluses. He is drowsy but arousable, respiratory rate is 14/min, SpO₂ is 96% on room air, and blood pressure is 135/88 mmHg. What is the most appropriate next step in management?