A 23-year-old tall slim man presents with sudden onset right-sided pleuritic chest pain and shortness of breath. He is a smoker but otherwise healthy. Examination reveals reduced chest expansion and decreased breath sounds on the right; he is haemodynamically stable. Chest X-ray shows a right-sided primary spontaneous pneumothorax with a 3 cm rim of air at the level of the hilum. What is the most appropriate initial management?