A 7-month-old boy presents with 2 days of coryzal symptoms, cough and increasing work of breathing. He has poor feeding but normal urine output. On examination, he has tachypnoea with subcostal recession and widespread fine crackles and wheeze. Oxygen saturation is 93% on room air. He is afebrile and otherwise well perfused. Which of the following is the most appropriate mainstay of management?