A 52-year-old male patient is 6 hours post-op following open abdominal hysterectomy. He is on intravenous patient-controlled analgesia (PCA) with morphine (1 mg bolus, 10-minute lockout interval, 4-hour limit of 30 mg). He reports inadequate pain relief despite pressing the button frequently. His vital signs are stable (BP 128/82, HR 88, RR 16, SpO₂ 98% on room air), and he is alert and oriented. What is the most appropriate next step in management?