A 68-year-old man admitted with pneumonia develops a rise in creatinine from 90 µmol/L to 190 µmol/L over 48 hours. Urine output has fallen to 0.3 mL/kg/hour for the past 6 hours. He is taking Ramipril and regularly using ibuprofen. Blood pressure is 95/60 mmHg. What is the most appropriate immediate management?