Will the father be sinful if he swore that his son would not be able to quit smoking immediately, and then the son quit?

There is no sin upon your father if you quit smoking immediately, even if he swore that you are unable to do so, because his oath was based on his assumption, and we have clarified that one who swears to something believing it to be true, but it turns out to be otherwise, does not incur sin. What is obligatory upon you is to quit smoking immediately, in repentance to Allah the Most High.