A 32-year-old man from Delhi presents with a 3-week history of productive cough, fever, and night sweats. Chest X-ray shows a cavitary lesion in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli (AFB). He is diagnosed with pulmonary tuberculosis and started on standard first-line anti-TB therapy (HRZE). After 2 months of treatment, repeat sputum smear microscopy remains positive. What is the most appropriate next step in management?