A 28-year-old man with a history of alcohol use disorder presents with a 3-day history of severe vomiting and abdominal pain. He has not eaten for 48 hours. On examination, he is lethargic and has signs of dehydration. Laboratory investigations show: blood glucose 42 mg/dL, serum lactate 8.2 mmol/L (normal <2), serum alanine 0.3 mmol/L (normal 0.4–0.8), and elevated liver enzymes (AST 320 U/L, ALT 180 U/L). What is the most appropriate immediate next step in management?