A 52-year-old man with a known diagnosis of Addison disease (on stable replacement with prednisolone 5 mg daily and fludrocortisone 0.1 mg daily) presents to the emergency department with acute-onset severe abdominal pain, vomiting, and diarrhea following a 2-day history of upper respiratory tract infection. His blood pressure is 100/65 mmHg (baseline 115/75 mmHg), heart rate 110/min, and he appears acutely unwell. Serum sodium is 132 mEq/L and potassium is 5.2 mEq/L. What is the most appropriate immediate management?