A 58-year-old man with advanced gastric cancer is admitted for palliative care. He is on morphine 20 mg 6-hourly for pain and complains of persistent nausea and vomiting. His renal function is normal (eGFR 78 mL/min/1.73 m²), and he is not on any other medications. The palliative care team wants to add an antiemetic that is particularly effective for opioid-induced nausea and vomiting (OINV). Which agent is most appropriate?