A 52-year-old man with type 2 diabetes mellitus presents with a 3-month history of progressive polyuria, polydipsia, and weight loss despite being on metformin 1000 mg twice daily for the past 2 years. His current HbA1c is 9.2% (target <7%). Fasting blood glucose is 185 mg/dL and postprandial glucose is 245 mg/dL. He has no contraindications to any antidiabetic agent. What is the most appropriate next step in management?