A 52-year-old man with a 10-year history of type 2 diabetes mellitus on metformin and glipizide presents with persistent nausea, fatigue, and a fruity odor on his breath. Laboratory findings show: blood glucose 320 mg/dL, pH 7.28, HCO₃⁻ 14 mEq/L, serum osmolality 340 mOsm/kg, and serum ketones mildly elevated (β-hydroxybutyrate 3.5 mmol/L). Urinalysis shows 3+ glucose and 1+ ketones. The patient is alert and oriented. What is the most appropriate immediate next step?