A 38-year-old man presents to the emergency department with severe epigastric pain radiating to the back for 6 hours. He reports consuming 60 mL of alcohol the previous night. On examination, he is diaphoretic with a blood pressure of 110/70 mmHg and heart rate of 105/min. Abdominal examination reveals epigastric tenderness without guarding. Laboratory investigations show: serum amylase 850 U/L (normal <100), lipase 1200 U/L (normal <60), WBC 14,500/μL, glucose 180 mg/dL, calcium 8.2 mg/dL, and LDH 520 U/L. Abdominal ultrasound shows no gallstones and a normal common bile duct. What is the most appropriate immediate next step in management?