A 25-year-old man with type 1 diabetes presents with abdominal pain, vomiting and Kussmaul respirations. Vitals: BP 92/58 mm Hg, HR 124 beats/min, RR 30 breaths/min, SpO2 99% on room air. Capillary glucose is 25 mmol/L, venous pH 7.10, serum ketones are elevated. According to Diabetes Canada, what is the single most appropriate first therapeutic step in managing this patient's diabetic ketoacidosis in the emergency department?