A 60-year-old man with a new diagnosis of Type 2 Diabetes has an HbA1c of 68 mmol/mol. He is started on metformin. Three months later, despite good adherence and lifestyle changes, his repeat HbA1c is 62 mmol/mol. He has a BMI of 31 kg/m². According to NICE NG28, what is the most appropriate second-line agent to add if metformin alone is not sufficient and his HbA1c remains ≥58 mmol/mol (or other locally agreed target)?