Firstly, just because he claims he traveled alone "many times" doesn't mean he actually did. Not only that but even if he has, that doesn't make it right and this time he has not been able to.
Maybe his eyesight has deteriorated, maybe as you say he' got away with it in the past. Without passing judgement on this case I've certainly been aware of a few rare instances where it has become clear after departure that a PRMs disability has been somewhat more severe than was "advertised", causing problems for both the cabin Crew and other passengers..