A 54-year-old man presents with vomiting and diarrhoea for 3 days. He has been taking Ramipril and Ibuprofen. His BP is 95/60 mmHg, pulse 110 bpm. Urine output has decreased. Serum creatinine has risen from 90 µmol/L (baseline 2 weeks ago) to 210 µmol/L. What is the most appropriate initial management?