A 3-year-old boy from rural Maharashtra presents to the emergency department with a 2-day history of acute watery diarrhea and vomiting. On examination, he is lethargic, with sunken eyes, absent skin turgor, weak radial pulses, and cold extremities. Heart rate is 140/min, respiratory rate 36/min, and blood pressure is 70/50 mmHg. Capillary refill time is >3 seconds. Serum sodium is 128 mEq/L, potassium 5.8 mEq/L, and pH 7.28 with HCO₃⁻ 12 mEq/L. What is the most appropriate immediate management?