A 25-year-old man with type 1 diabetes presents with nausea, abdominal pain and polyuria after missing several insulin doses. Vitals: HR 112, BP 104/66 mm Hg, RR 26, T 37.8 °C. Capillary glucose is 23 mmol/L, venous pH 7.19, bicarbonate 12 mmol/L and serum ketones are elevated. What is the most appropriate initial management step in the emergency department?