A 68-year-old man, known to be taking ramipril 10mg daily for hypertension, presents to the Emergency Department after 2 days of profuse vomiting and diarrhoea. He feels dizzy and has passed minimal urine today. On examination, his BP is 95/60 mmHg, HR 110 bpm, and mucous membranes are dry. Blood tests show: Urea 28 mmol/L, Creatinine 310 µmol/L (baseline 115 µmol/L). What is the most important immediate management step regarding his medication?