Hi
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Normally there is no contraindication to flying, especially a year after head injury. Since there are no new neurological symptoms, it should be safe. However, since he has headaches (I understand they are persisting since before the accident), please get this investigated at the earliest opportunity. It could be something as simple as refractive error of eye like myopia.
Hope this helps. Take care!
No. There is no danger from the cabin pressurization on a transatlantic flight.