The left trigger will be iron-sights or scope depending on how far you pull the trigger. Please make sure he is pressing the trigger right down for the scope.
My son also has the same issue, it did work and now doesn't
My son has been unable to scope in game after he saved and quit earlier. Pressing the LT only brings up the crosshairs. He has tried to reset all controls to default, deleted his saved game and still the issue remains. He is now uninstalling and having to re-download the game to try and see if that works.
Anyone else have this issue and if so, how did you fix it?