What about provision that addresses the potential issue rather than just giving 10 minutes more…
Does he have slow writing, is he more proficient at typing and could use a laptop?
Is he experiencing anxiety so doesn’t use his time effectively – panicking over questions rather than answering them. Would he benefit on a 2min rest break between questions which – if used effectively it could potentially allow him the time to calm, refocus and complete the next question.
And if needed, he could potentially have a combination of the above and if qualifying the extra time too 😉