A 35-year-old man presents with a 3-month history of increased thirst, polyuria and 6 kg unintentional weight loss. Fasting plasma glucose is 12.4 mmol/L on two separate occasions and HbA1c is 78 mmol/mol (9.3%). He has no ketones or acidosis. According to the RACGP/Diabetes Australia handbook, what is the most appropriate initial pharmacological therapy in addition to lifestyle modification?