A 75-year-old male with a 30-year history of type 2 diabetes mellitus (T2DM) developed proteinuria and hypertension (HTN) over the past 6 years, with progressive renal dysfunction over the last 2 years. For the past 3 months, he has experienced nausea and vomiting, along with bilateral lower extremity edema. Recent laboratory tests show serum K+ 5.5 mmol/L, CO2CP 16.5 mmol/L, Cr 830 μmol/L, Hb 6.5 g/dL, and blood glucose 16 mmol/L. He has a history of subtotal gastrectomy. The patient's current renal function is at which stage?