What is the ruling regarding a father going to a sorcerer and doing what he ordered him to do, such as bathing on a grave, due to the successive deaths of his children, especially given his prior weak religious commitment?

Sorcery is a gateway to disbelief (kufr), and the sorcerer is a disbeliever (kafir), as are the soothsayer (kahin) and the astrologer (munajjim). Whoever goes to a sorcerer, soothsayer, or astrologer and believes him, has disbelieved in what was revealed to Muhammad. Your father, by committing this act, has sinned, incurred guilt, and committed a major sin. He must repent to Allah with sincere repentance, for through repentance, Allah forgives all sins. If your father has passed away, then seek forgiveness for him, supplicate for him, and give charity on his behalf.