A 45-year-old man with known primary adrenal insufficiency (Addison disease) presents with vomiting, abdominal pain and profound weakness. He stopped taking his Hydrocortisone 3 days ago. Observations: blood pressure 80/50 mmHg, pulse 120 beats/min, sodium 124 mmol/L, potassium 6.0 mmol/L, glucose 2.9 mmol/L. What is the most appropriate immediate treatment?