A 52-year-old man from Delhi presents with a 3-month history of polyuria, polydipsia, and weight loss of 4 kg. He is a known hypertensive on amlodipine. On examination, BMI is 28 kg/m², blood pressure 138/88 mmHg. Fasting blood glucose is 156 mg/dL, postprandial glucose 248 mg/dL, and HbA1c is 8.2%. Renal function and liver function tests are normal. He is started on metformin 500 mg twice daily with dietary modifications. After 6 weeks, fasting glucose is 132 mg/dL and postprandial glucose is 210 mg/dL. His HbA1c remains 8.1%. What is the most appropriate next step in pharmacological management?