A 3-month-old male infant from rural Maharashtra presents with a 2-week history of paroxysmal cough followed by a characteristic high-pitched inspiratory sound. The mother reports the infant has not received any vaccinations. On examination, the child is afebrile, with copious nasal secretions and mild conjunctival injection. Chest examination reveals scattered crackles but no consolidation. A nasopharyngeal swab culture on Bordet-Gengou medium shows small, gram-negative coccobacilli. What is the most appropriate next step in management?