All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If your father had missed fasting due to an incurable disease or due to his old age, then in this case, you have to feed a poor person for every day he missed fasting as you did during his lifetime. Therefore, you are not obliged to do anything after his death. On the other hand, if the disease was curable but it continued till his death, then in this case, you are free from any obligation and the feeding of the poor person that you had done is considered only a good act, you will surely be rewarded for that.
Allah knows best.