A 6-year-old boy is receiving a continuous morphine infusion via a patient-controlled analgesia (PCA) device after major abdominal surgery. On the second postoperative day, he develops persistent itching on his chest and face, which is distressing him. His pain is well-controlled, and he is slightly drowsy but easily arousable. Which intervention is most appropriate to manage his pruritus without compromising his analgesia?