A 4-year-old boy with a history of febrile seizures is brought to the emergency department by his mother. He has been actively seizing for the past 18 minutes. His temperature is 39.2°C, heart rate 140/min, respiratory rate 28/min, and oxygen saturation 94% on room air. He is unresponsive to verbal commands. His mother reports he received paracetamol 30 minutes ago but the seizure did not stop. What is the most appropriate immediate next step in management?