A 24-year-old tall, thin man presents with sudden onset left-sided pleuritic chest pain and mild dyspnoea. He is a smoker but otherwise healthy. On examination he has reduced breath sounds over the left apex; vitals are stable with SpO₂ 98% on room air. Chest X-ray shows a small left primary spontaneous pneumothorax with a 1 cm rim of air at the apex. What is the most appropriate initial management?