A 7-year-old boy from Chennai presents with fever for 5 days, severe myalgia, and headache. On examination, he has a petechial rash on the trunk and a positive tourniquet test. His platelet count is 85,000/μL, haemoglobin 14 g/dL, and haematocrit 38%. Dengue serology (IgM) is positive. His blood pressure is 98/62 mmHg, heart rate 110/min, and capillary refill time is 2 seconds. What is the most appropriate immediate management?