A 62-year-old man presents with sudden onset severe epigastric pain radiating to the back, associated with vomiting. He drinks 6-8 beers daily. Serum amylase and lipase are markedly elevated, and ultrasound shows no gallstones. His pain is uncontrolled with regular paracetamol and oral oxycodone. Which intravenous analgesic is most appropriate for acute severe pain in pancreatitis in the emergency department?