A 45-year-old man from Delhi with a 6-month history of progressive fatigue, dyspepsia, and intermittent melena is found to have Hb 8.5 g/dL, MCV 72 fL, serum ferritin 18 ng/mL, and TIBC 410 µg/dL. He has been taking oral ferrous sulfate 200 mg daily for 3 weeks but Hb has not changed. What is the most appropriate next step in management?