A 18-month-old boy from rural Maharashtra presents with a 3-day history of watery diarrhea (8–10 stools per day) and vomiting. On examination, he appears lethargic, with sunken eyes, dry mucous membranes, and skin turgor that returns slowly (>2 seconds). Capillary refill time is 2.5 seconds. Weight is 11 kg (baseline 12 kg). Serum sodium is 128 mEq/L, potassium 3.2 mEq/L, and bicarbonate 15 mEq/L. What is the most appropriate fluid regimen for this child?