Is it obligatory for a cancer patient who has undergone surgery, suffers from chronic diseases, and is elderly to make up for missed fasts (qada') or to feed the poor (fidya), given his intention to fast next Ramadan?

Your father should feed a poor person for each day of missed past Ramadans and future ones, if he is unable to fast due to old age and illness. This is obligatory according to the majority of scholars and recommended according to the Malikis. If he recovers and becomes able to fast, he is obligated to make up the missed fasts according to the Hanafis. However, according to the majority of scholars, he is not obligated to make up the missed fasts but should fast the upcoming ones.