A 35-year-old man with a history of seasonal allergic rhinitis presents with acute onset of severe dyspnea, wheezing, and chest tightness 20 minutes after inhaling dust from stored grain in his warehouse. He is alert, SpO₂ 88% on room air, and peak expiratory flow is 40% of predicted. He has no prior anaphylaxis. What is the most appropriate immediate next step in management?