This will be a result of VH-VKJ remaining on the ground at KIX (see above). This is its eighth full day inoperative, punching a hole in aircraft availability. Completely unplanned!
However on at least one afternoon (see above), a QF B744 substituted ex SYD. I have not researched to see if this is still occurring.
Pushka, was your son offered an alternative flight at no cost with rebooking by JQ? Which airline? If not, was he given an immediate full refund?
AFF member MEL_Traveller previously asserted that JQ (always) lacks sufficient B788s to operate its published timetable. That would be unusual, but I'm not doubting 'MEL_' as many other of his posts are 105 per cent correct or arguable.