A 52-year-old man with bipolar disorder has been on lithium 900 mg daily for 3 years with good mood stabilization. He presents to the emergency department with tremor, confusion, ataxia, and slurred speech for 6 hours. His wife reports he had acute gastroenteritis with vomiting and diarrhea for 2 days and continued taking lithium. Serum lithium level is 2.8 mEq/L (therapeutic range 0.6–1.2 mEq/L). Serum creatinine is 1.4 mg/dL (baseline 1.0 mg/dL). What is the most appropriate immediate next step in management?