A 65-year-old man with known diabetic nephropathy has routine blood tests showing an eGFR of 28 ml/min/1.73m² and a serum potassium level of 5.8 mmol/L. His medications include Ramipril 10mg OD, Atorvastatin 40mg ON, and Metformin 1g BD. His ECG shows peaked T waves. What is the most appropriate immediate management step?