A 58-year-old man with hypertension is noted to have an estimated Glomerular Filtration Rate (eGFR) of 52 mL/min/1.73m^2 on routine blood tests. This is a new finding, previous eGFR >60. Urinalysis shows no protein or blood. Blood pressure is 138/84 mmHg on ramipril 5mg od. What is the most appropriate classification of his kidney function according to NICE guidelines?