A 24-year-old man with known sickle cell disease presents to the Emergency Department with severe pain in his back, chest, and limbs, rating it 9/10. This is typical of his vaso-occlusive crises. His temperature is 37.2°C, HR 100 bpm, RR 20/min, SpO2 97% on air. According to NICE guidelines, what is the most important immediate step in his management?