A 52-year-old man undergoes total abdominal hysterectomy for uterine fibroids under general anesthesia. On postoperative day 1, he is prescribed intravenous patient-controlled analgesia (IV-PCA) with morphine. The settings are: demand dose 2 mg, lockout interval 10 minutes, 4-hour limit 30 mg. At 6 hours postoperatively, the patient complains of inadequate pain relief despite pressing the button 8 times in the past hour. His respiratory rate is 18/min, oxygen saturation 96% on room air, and he is alert and oriented. What is the most appropriate next step in management?