A 28-year-old man presents with a 2-week history of polyuria, polydipsia, and weight loss of 3 kg. On examination, he appears acutely unwell with Kussmaul respiration. Blood glucose is 450 mg/dL, arterial pH 7.25, HCO₃⁻ 12 mEq/L, and serum ketones are positive. C-peptide is undetectable. What is the most appropriate immediate next step in management?