A 20-year-old is brought in by ambulance due to severe shortness of breath. He is known to be asthmatic and has been non-compliant with his steroid puffers but has used a full salbutamol inhaler since this morning. Vitals are BP 147/90, HR 133, RR 34, Sat 91% on room air, GCS 15. He is sitting in the tripod position and has both inspiratory and expiratory wheeze on exam. He is cooperative with your interventions but is not improving with your treatment protocol. You are considering positive pressure ventilation. Which modality will be most appropriate for him at this stage?