A 18-month-old boy from rural Maharashtra presents to the emergency department with a 3-day history of watery diarrhea (8–10 stools/day) and vomiting. On examination: sunken eyes, skin turgor returns slowly (>2 seconds), mucous membranes dry, urine output reduced. Vital signs: HR 140/min, BP 90/60 mmHg, RR 28/min. Weight 10 kg. The child is alert and able to drink. What is the most appropriate next step in management?