A 22-year-old man presents with sudden onset right-sided pleuritic chest pain and breathlessness. He is tall and thin, a smoker, with no past lung disease. Observations: BP 120/75 mmHg, respiratory rate 22/min, SpO2 97% on air. Chest X-ray confirms a small (20%) primary spontaneous pneumothorax without tension. He is comfortable at rest. What is the most appropriate initial management?