A 52-year-old man with bipolar disorder on lithium 900 mg daily for 3 years presents to the emergency department with tremor, confusion, and ataxia. His wife reports he has had diarrhea and vomiting for 2 days following gastroenteritis. Serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). Serum creatinine is 1.4 mg/dL (baseline 1.0 mg/dL), and eGFR is 52 mL/min/1.73 m². What is the most appropriate immediate next step in management?