Does the father's repentance, prayer, fasting, Hajj, and change of his previous life suffice him from asking about expiation for intentionally breaking his fast in Ramadan, and is he thereby on the right path?

We praise Allah for guiding your father. To answer your question, several matters need to be clarified:

1. Prayer (Salah): It is the second pillar of Islam. One who abandons it while denying its obligation is, by consensus, a disbeliever (kafir). As for one who abandons it out of laziness, there is a difference of opinion among scholars. If your father abandoned it out of laziness, he must make up all the prayers he missed since puberty. 2. Fasting (Sawm): He must make up the days of Ramadan he missed after puberty. If he intentionally engaged in sexual intercourse during a Ramadan day, he must offer a major expiation (kaffarah kubra), which is repeated for each day that intercourse occurred. 3. Adultery/Fornication (Zina): It is one of the gravest sins. Allah Almighty says: (And do not approach unlawful sexual intercourse. Indeed, it is ever an indecency and an evil way.) [Al-Isra 17:32]. In addition to this, seclusion with a non-mahram woman is forbidden.

You should advise your father with wisdom and gentleness, explaining what he needs to make up, while also emphasizing the vastness of Allah's mercy and forgiveness for those who sincerely repent. Try to share this fatwa with him and strive in supplicating for him.