A 24-year-old man with type 1 diabetes presents with nausea, vomiting and abdominal pain. He has been unwell with influenza. Vitals: temperature 37.9 °C, heart rate 118 beats/min, blood pressure 104/66 mm Hg, respiratory rate 26 breaths/min with Kussmaul breathing. Capillary glucose is 24 mmol/L, serum ketones are elevated and venous blood gas shows pH 7.20 and bicarbonate 12 mmol/L. What is the most appropriate initial management step?