A 52-year-old man with a 10-year history of type 2 diabetes is admitted with a blood glucose of 680 mg/dL, serum osmolality of 320 mOsm/kg, and pH 7.35 (mild acidosis). Serum ketones are negative. He is alert but confused, with signs of severe dehydration. IV fluids are initiated. What is the most appropriate immediate pharmacological intervention?