A 32-year-old man is brought to the emergency department 6 hours after ingesting approximately 100 mL of methanol-contaminated alcohol at a local distillery. He is conscious, alert, and complains of visual disturbances (blurred vision) and mild abdominal pain. Vital signs are stable. Serum methanol level is 45 mg/dL. Arterial blood gas shows pH 7.28, HCO₃⁻ 16 mEq/L, and normal anion gap at present. What is the most appropriate immediate next step in management?