My father fell ill and the physician advised him not to fast for a year. He gave food to an indigent for every day in Ramadhaan that he did not fast as expiation. The next Ramadhaan, he fasted. Now, is it due upon him to make up for the missed fasts of the last Ramadhaan or is expiation only sufficient?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, , is His Slave and Messenger.

What appears from the question is that when your father could not fast Ramadan because of his ailment, he knew that his ailment was curable.

Hence, it is due on him, according to the agreement of scholars to make up for the fasts of that month as his ailment was curable. Note that Allaah The Almighty Says (what means): {So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up].}[Quran 2:184]

Making up these fasts are due upon him and it is incumbent upon him to make up for the fasts that he could not observe. If he delayed making up intentionally without a valid excuse until the following Ramadan came, then it also becomes incumbent upon him to give food to an indigent for every missed day that he delayed making up for. This ruling is based on the Fatwa that was passed by Ibn ‘Abbaas and Abu Hurayrah in this regard. It is worth-mentioning that their Fatwa was not objected to by any of the Companions . 

As regards the food that he has already given to indigent persons, it does not suffice him because he had given that food before expiatory feeding became due on him. Note that giving expiatory food became obligatory on him due to the fact that he delayed, for a whole year, the fasts that he had to make up for.

However, this ruling applies to him if he knew that it was forbidden for him to delay the making up for the missed fasts until the advent of the next Ramadan, but if he was ignorant of that, then he will be exempted from expiating.  

Allaah Knows best.