A 72-year-old man presents to the emergency department with a 2-day history of productive cough, fever, and confusion. His observations give a NEWS2 score of 8: temperature 38.6°C, respiratory rate 28/min, oxygen saturations 90% on air, blood pressure 88/52 mmHg, heart rate 118 bpm. You suspect sepsis secondary to pneumonia. What is the single most appropriate immediate management step?