Mokkah suffered a spontaneous pneumothorax when he was about 18 months old and training to race. He was monitored to see of the hole would close up by itself, but it didn't---he did end up having emergency surgery to repair it.
They biopsied the lung and came up with nothing---they still don't know why or how it happened, or if it would happen again---but if it's any comfort to you, he's now over 4 years old and in perfect health.
If you want to get advice from vets who have experience in treating this condition, I'd be happy to give you the names and phone numbers of the vets here who treated him.