A 35-year-old man from Mumbai presents on day 6 of fever with sudden onset of severe abdominal pain, persistent vomiting, and lethargy. On examination, he is restless, blood pressure 98/64 mmHg, heart rate 108 bpm, and has a positive tourniquet test with petechiae on the lower limbs. Laboratory investigations show WBC 2800/μL, platelets 45,000/μL, hematocrit 48% (baseline 42%), AST 520 U/L, and albumin 3.0 g/dL. What is the most appropriate immediate management?