A 5-year-old boy is brought to the Emergency Department with a 2-hour history of noisy breathing and a harsh, barking cough. His mother reports he had a mild cold for the past day. On examination, he is alert, with inspiratory stridor at rest, mild intercostal retractions, and a respiratory rate of 30/min. His oxygen saturation is 96% on room air. What is the most appropriate immediate pharmacological management?