A 58-year-old man with chronic obstructive pulmonary disease (COPD) and sleep apnea undergoes open reduction and internal fixation of a femoral fracture under spinal anesthesia. Postoperatively, he is prescribed intravenous PCA with morphine (1 mg bolus, 10-minute lockout, 4-hour limit of 30 mg). On postoperative night 1, the nursing staff finds him drowsy but arousable, with a respiratory rate of 10 breaths/minute, oxygen saturation of 92% on 2 L/min nasal cannula, and pinpoint pupils. He has pressed the PCA button only 3 times in the last 4 hours. What is the most appropriate immediate intervention?