A 18-month-old boy from rural Maharashtra presents to the emergency department with a 3-day history of watery diarrhea and vomiting. His mother reports he has passed 8–10 loose stools per day for the past 2 days. On examination, he appears lethargic, his anterior fontanelle is sunken, skin turgor is decreased (skin pinch goes back in >2 seconds), and mucous membranes are dry. Capillary refill time is 2.5 seconds. His weight is 11 kg (baseline 12 kg). Serum sodium is 128 mEq/L, potassium 3.2 mEq/L, chloride 98 mEq/L, and HCO₃⁻ is 12 mEq/L. What is the most appropriate immediate management?