A 21-year-old man with type 1 diabetes presents with 24 hours of polyuria, vomiting and abdominal pain. He has missed several insulin doses. On examination, he is tachycardic, tachypnoeic with Kussmaul respirations, and dehydrated. Capillary glucose is 28 mmol/L and blood ketones are 6 mmol/L. What is the most appropriate immediate management step?