A 7-year-old boy with known asthma presents to the emergency department with severe dyspnoea, inability to speak in full sentences, respiratory rate 36/min, SpO₂ 90% on room air, and diffuse wheeze with poor air entry. After initial nebulised Salbutamol and Ipratropium plus systemic corticosteroids, he remains in severe distress. According to Canadian asthma guidance, what is the next most appropriate step?