A 24-year-old tall, slim man with no past medical history presents with sudden onset right-sided pleuritic chest pain and breathlessness. He is stable with respiratory rate 22/min, BP 120/75 mmHg and oxygen saturation 97% on air. Chest X-ray confirms a right primary spontaneous pneumothorax with a visible rim of 3 cm between lung edge and chest wall at the level of the hilum. He is breathless. What is the most appropriate initial management?