How is the expiation for fasting due from a father who did not fast for many years due to drugs and intoxicants, then repented and became ill, unable to fast, to be paid? And are the days counted from his youth or from his repentance? How is the expiation calculated? And is it permissible for the son to pay it on behalf of his poor father?

Your father must make up the days he missed from Ramadans since puberty. If he is continuously unable to make them up due to old age or an incurable illness, then he must feed one poor person for each day, with a "mudd" of food.

If he was unaware of the impermissibility of delaying (the make-up fasts), then there is no expiation upon him. However, if he knew of the impermissibility of delaying the make-up fasts until after the following Ramadan, then he must pay expiation for each day with a "mudd" of food for each poor person.

Your father is considered capable of paying the expiation if he possesses sustenance that exceeds his and his dependents' sustenance for a day and a night. If he finds what he can give out in excess of his needs, then he must give it out. Otherwise, it remains a debt upon him until he finds what he can give out, and it has also been said that it is waived for him.