What is the fate of a deceased father who prayed infrequently (he would pray Maghrib and sometimes not pray it at all), and who passed away in the middle of the month of Ramadan, and what can his son do for him?

The one who abandons prayer out of laziness is in grave danger. As long as there is no consensus on his disbelief, we hope for mercy and forgiveness for him. There is no objection to giving charity on his behalf, seeking forgiveness for him, and asking Allah for the forgiveness of his sins and the overlooking of his missteps.