Don't smoke. Non-trauma, spontaneous pneumothorax (sp) occur as a result of an abnormality in the lung tissue. Risk factors for primary and secondary sps include: smoking (including marijuana), tall thin stature in otherwise healthy, connective tissue disorders, pregnancy, familial history. Sps are associated with 20-40% recurrence rate (depending type).
Collapsed lung. Did it occur as a result of trauma? Or did it happen spontaneously? Is he a smoker? Smokers can develop emphysema, in which the air sacs coalesce and lung tissue "thins out" making it easy for a sac to burst; if it's on the lung margin, the lung can collapse. Make sure he stops smoking and is not exposed to second-hand smoke.