A 38-year-old man with schizophrenia has been on risperidone 6 mg/day for 2 years with good symptom control. He now presents with tremor, rigidity, and hyperthermia (38.5°C). His creatine kinase is 1200 U/L, and serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL). What is the most appropriate immediate next step?