A 58-year-old male with COPD is admitted to the ICU with acute exacerbation and hypercapnic respiratory failure (pH 7.28, PaCO₂ 65 mmHg, PaO₂ 55 mmHg on FiO₂ 0.4). He is intubated and placed on volume-controlled ventilation with a tidal volume of 8 mL/kg, rate 16/min, and PEEP 5 cm H₂O. After 2 hours, arterial blood gas shows pH 7.32, PaCO₂ 58 mmHg, PaO₂ 72 mmHg. The patient is triggering the ventilator appropriately with good chest rise. What is the most appropriate next step in management?