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, blood pressure is 128/82 mmHg, and there is no clinical evidence of volume depletion or edema. Laboratory investigations reveal: Na⁺ 118 mEq/L, K⁺ 4.2 mEq/L, Cl⁻ 102 mEq/L, HCO₃⁻ 24 mEq/L, urine osmolality 580 mOsm/kg, serum osmolality 245 mOsm/kg, and urine Na⁺ 65 mEq/L. What is the most appropriate immediate management?