A 25-year-old man presents with sudden onset sharp left-sided pleuritic chest pain and dyspnoea. He is tall and thin, a smoker, and has no other medical history. He is haemodynamically stable with SpO₂ 98% on room air. Chest X-ray shows a small left primary spontaneous pneumothorax with a 1.5 cm rim at the hilum. What is the most appropriate management?