A 6-year-old boy with known asthma presents to the emergency department with a 2-hour history of wheeze and increased work of breathing after a viral illness. He is speaking in short sentences, respiratory rate 32 breaths/min, oxygen saturation 95% on room air and pulse 120 beats/min. According to Australian paediatric acute asthma guidelines, which is the preferred method for delivering initial short-acting beta agonist therapy?