First I would ask do you mean he still can't see out of the eye? If that IS THE CASE a cause must be found. If they have looked in the eye and the inside looks normal then the problem is behind the eye and he may need special tests like MRI, angiograms, etc.
If the vision has come back he may have had a TIA (transient ischemic attack) and he needs to see a internist or cardiologist and will need full work up when involves examining heart, blood vessels, blood, brain.
The best thing this could be if his vision is back to normal is a "migraine with aura"
Keep looking for answers