A 58-year-old man with a history of small-cell lung cancer presents to the emergency department with confusion and lethargy. His wife reports he has been increasingly drowsy over the past 3 days. On examination, he is disoriented to time and place, with no focal neurological deficits. Blood pressure is 128/82 mmHg, heart rate 88/min, and respiratory rate 18/min. Laboratory investigations reveal: Na⁺ 118 mEq/L, K⁺ 3.8 mEq/L, Cl⁻ 102 mEq/L, HCO₃⁻ 24 mEq/L, urine osmolality 680 mOsm/kg, serum osmolality 240 mOsm/kg, and urine Na⁺ 65 mEq/L. What is the most appropriate initial management?