Is it obligatory for someone who broke their fast while young during their first Ramadan, being unable to fast, to make up the days (qada') and offer expiation (fidyah)? And is it permissible to delay making up the days until just before the next Ramadan, fasting them intermittently?

The fatwa issued by your father is incorrect, and it is not permissible for a mature Muslim to intentionally break their fast in Ramadan without a legitimate excuse, such as travel or illness. Therefore, you must make up the month you intentionally missed without a valid excuse.

You must make up the missed days immediately without delay, especially since the breaking of the fast was intentional and without excuse. This is the view of the Khurasanis and some Iraqis.

In addition to making up the fast immediately, you must feed a poor person for each day you delayed making up the fast. This is the opinion of the majority of scholars.