A 24-year-old tall, slim man with no past medical history presents with acute onset right-sided pleuritic chest pain and breathlessness. He is haemodynamically stable. Chest examination reveals reduced expansion and reduced breath sounds on the right. A chest X-ray confirms a right-sided primary spontaneous pneumothorax with a 1.5 cm rim of air between the lung margin and chest wall. What is the most appropriate initial management?