A 68-year-old man with COPD is admitted with an acute exacerbation. He is using accessory muscles, RR 32/min, and SpO2 86% on room air. Arterial blood gas on 2 L/min oxygen via nasal cannula shows pH 7.28, PaCO2 70 mm Hg, PaO2 58 mm Hg, HCO3- 32 mEq/L. What is the most appropriate next step in ventilation support?