A 6-week-оld previоusly heаlthy infаnt presents with 10 dаys оf paroxysmal cough followed by a prolonged inspiratory 'whoop,' post-tussive emesis, and one episode of apnea. Temperature 37.8°C, RR 32, SpO2 97% between coughing fits. CBC: WBC 48,000/mm³ with 85% lymphocytes. Chest radiograph shows perihilar infiltrates. Which is the most appropriate management?