A 6-year-old boy presents to the emergency department with a 3-day history of increasing abdominal pain, fever, and frequent watery, sometimes bloody, stools. His symptoms began approximately 8 days after completing a 10-day course of oral amoxicillin for pharyngitis. He appears lethargic and dehydrated, and his mother reports decreased urine output. On physical examination, his temperature is 39.8 C (103.6 F), pulse is 110/min, and blood pressure is 80/50 mmHg. Abdominal examination reveals diffuse tenderness with rebound tenderness, particularly in the lower quadrants. Bowel sounds are diminished. Given these findings, what is the most appropriate initial diagnostic approach and immediate management strategy?