A 72-year-old man is found on routine blood tests to have an eGFR of 45 mL/min/1.73 m² on two occasions 3 months apart and a urine albumin-creatinine ratio (ACR) of 8 mg/mmol. According to Kidney Health Australia's CKD Management in Primary Care handbook, how should his kidney disease be staged and what is the most appropriate next step?