A 52-year-old man with obesity (BMI 31 kg/m²) is diagnosed with type 2 diabetes mellitus after a fasting plasma glucose of 8.9 mmol/L and an HbA1c of 7.8%. His creatinine is 90 µmol/L (eGFR 78 mL/min/1.73 m²), and he has no cardiovascular disease. Despite 3 months of diet and exercise, his HbA1c is unchanged. What is the most appropriate next step in pharmacologic management?