A 40-year-old man with known primary adrenal insufficiency on maintenance hydrocortisone and fludrocortisone presents to the emergency department with vomiting, diarrhoea and abdominal pain for 24 hours. He is lethargic, blood pressure is 86/54 mm Hg, heart rate 118 beats/min and he appears volume depleted. Which is the most appropriate immediate management step?