A 6-year-old boy from rural Maharashtra presents with acute onset watery diarrhea, abdominal cramps, and fever for 2 days. Stool microscopy shows RBCs and WBCs. Culture on HE agar shows non-motile, lactose-fermenting colonies. Biochemical tests confirm Shigella flexneri. The child is moderately dehydrated but hemodynamically stable. What is the most appropriate immediate next step in management?