A 32-year-old man from rural Maharashtra presents with a 3-month history of cough, fever, and night sweats. Chest X-ray shows cavitary lesions in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli. He is started on standard first-line anti-TB therapy. After 2 months of treatment, repeat sputum smear microscopy remains positive despite good adherence. What is the most appropriate next step in management?