How and when is a father's ablution (wudu') invalidated after the removal of the rectum and the diversion of the faecal matter to an opening in the abdomen, given his inability to control excretion or sense the emission of gas?

Your father must perform Wudu for every prayer after its time has begun. He is not harmed by any stool or gas that exits him during or after the prayer. He is excused for carrying the bag of impurities during prayer. He should, as much as possible, take precautions to prevent impurities from touching his body or clothes.