A 35-year-old male construction worker from rural Maharashtra presents with a 3-week history of productive 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 (AFB) on two consecutive samples. He is started on anti-TB therapy under RNTCP. After 2 months of treatment, repeat sputum smear microscopy remains positive. What is the most appropriate next step in management according to NTEP guidelines?