A friend of mine told me that his parents married in Ramadan. His father believed wrongly that fasting was not obligatory for him and his wife as long since they were newlyweds. Accordingly, they did not fast the entire Ramadan in which they got married. The mother stated she made up the days owed, but the father died before doing so. The question is: can the son make up the missed days on behalf of his father? Are there any other requirements?

Assuming that the man mentioned broke his fast by eating and drinking, and made no intention to fast out of his mistaken belief that he did not need to because he was a newlywed: the ruling in this case is that one must make up the missed days, though an atonement is not necessary. It is true that he necessarily had sexual intercourse with his wife but he did not make the intention to fast to start with.

His son must feed a poor person for each missed day of fasting. This means that he has to feed 30 poor persons on behalf of his father.
And Allah Most High knows best.