A 28-year-old man from Bangalore presents on day 7 of illness with fever that defervesced 2 days ago. He now complains of severe epigastric pain, persistent vomiting, and dizziness on standing. On examination, his blood pressure is 100/68 mmHg (sitting) and 88/62 mmHg (standing), pulse 112/min, and capillary refill time is 2.5 seconds. Platelet count is 45,000/μL, haematocrit has risen from 38% to 44% over 24 hours, and AST is 280 U/L. There is no active bleeding. What is the most appropriate immediate management?