A 19-year-old man with known Type 1 Diabetes presents with vomiting, abdominal pain, and confusion. His breath smells of ketones. Blood gas analysis shows pH 7.15, bicarbonate 12 mmol/L. Capillary blood glucose is 25 mmol/L. Capillary ketones are 4.5 mmol/L. What is the most crucial initial step in management?