A 3-day-old male neonate, born to a 28-year-old primigravida mother with prolonged rupture of membranes (PROM) for 18 hours, presents with poor feeding, lethargy, and a temperature of 38.2°C (rectal). On examination, the infant appears ill with mild abdominal distension and hepatomegaly. Blood culture is pending. Complete blood count shows WBC 4,200/μL (reference: 5,000–21,000), platelets 85,000/μL, and CRP 24 mg/L. What is the most appropriate initial management?