A 55-year-old man with known type 2 diabetes, managed with metformin 1g BD, attends for his annual review. His HbA1c is 68 mmol/mol (target is 48-53 mmol/mol). His eGFR is >60 ml/min/1.73m², he has no history of cardiovascular disease, heart failure, or chronic kidney disease. According to NICE NG28, what is the most appropriate next step in his pharmacological management?