A 3-month-old boy is brought to A&E with a 2-day history of coryzal symptoms, poor feeding and increasing work of breathing. He was born at term and is normally well. On examination he has tachypnoea (RR 60 breaths/min), subcostal recession and widespread fine inspiratory crackles and wheeze. Oxygen saturation is 91% on air and he is taking about half his usual feeds. What is the most appropriate management?