A 58-year-old man with osteoarthritis is started on an ACE inhibitor for hypertension and takes regular ibuprofen for joint pain. One week later he develops reduced urine output and fatigue. Creatinine has risen from 85 to 210 µmol/L. Blood pressure is 110/70 mmHg and there is no urinary obstruction on ultrasound. What is the most appropriate next step?