Petechiae — Meningococcemia / DIC

A 4-year-old boy presents to the emergency department with a 6-hour history of fever (39.8°C), irritability, and neck stiffness. On examination, you notice multiple small, dark red lesions on his trunk and lower extremities that do NOT fade when you press a clear glass slide against them. The lesions marked **A** in the diagram are consistent with petechiae. Which of the following is the most appropriate IMMEDIATE management step?