A 68-year-old man with a 3-year history of progressive memory loss and difficulty recognizing family members is brought to the hospital by his son. Over the past 48 hours, he has become acutely confused, agitated, and is seeing insects crawling on his skin. His vital signs show a temperature of 38.5°C, heart rate 110/min, and respiratory rate 24/min. On examination, he is disoriented to time and place, with fluctuating levels of consciousness. His son reports he was on cough syrup containing dextromethorphan for the past 3 days. What is the most appropriate immediate next step in management?