A 32-year-old male patient is undergoing elective abdominal surgery under general anesthesia. After induction with propofol and fentanyl, succinylcholine 1.5 mg/kg IV is administered for rapid sequence intubation. Within 30 seconds of administration, the patient develops severe muscle rigidity, trismus, and a rapid rise in core body temperature. Arterial blood gas shows pH 7.28, PaCO₂ 58 mmHg, and K⁺ 6.8 mEq/L. What is the most appropriate immediate next step in management?