5 months post-op, I would expect that most of a C3F8 gas to have been reabsorbed, but there may have still been a few bubbles left which could have expanded. And if he is able to see in another position, usually facedown, than gas
expansion could be a likely explanation for his symptoms. If SF6 was used it should have all been absorbed, so gas expansion would not likely be the problem.
Either way, he needs to be examined by an ophthalmologist!