A 22-year-old man with type 1 diabetes presents with vomiting and abdominal pain. He has been unwell for 24 hours and omitted his insulin. On examination he is drowsy, tachypnoeic with Kussmaul respirations, heart rate 120 beats/min, blood pressure 100/60 mmHg. Venous blood gas shows pH 7.12, bicarbonate 8 mmol/L, glucose 28 mmol/L and ketones 6.0 mmol/L. What is the single most appropriate immediate management step?