Is my father bound by an oath expiation (kaffarat yameen) if I quit smoking immediately and permanently, after he swore that I would not be able to quit except gradually?

Scholars have differed regarding one who swears by what they believe to be true (ghalabat al-dhann) concerning a future matter, and it turns out to be mistaken: is expiation (kaffarah) required of them or not? The more cautious opinion is that it is required, and this is the view of the majority. So, if you were to quit smoking immediately, expiation would be incumbent upon your father according to the majority of scholars, but it would not be incumbent upon him according to the choice of Sheikh al-Islam Ibn Taymiyyah. It is obligatory upon you to promptly abandon smoking and repent from it immediately, as it is among the forbidden acts.