A 32-year-old male patient is scheduled for emergency appendicectomy under general anesthesia. He has a history of malignant hyperthermia (MH) in a first-degree relative. During induction, the anesthesiologist avoids volatile anesthetics and uses propofol with nitrous oxide. For muscle relaxation, succinylcholine is administered at 1.5 mg/kg IV. Within 2 minutes of drug administration, the patient develops severe muscle rigidity, core temperature rises to 39.2°C, end-tidal CO₂ increases to 65 mmHg, and urine becomes dark brown. What is the most appropriate immediate management?