Must one make up the day if one replies to insults during the daytime in Ramadan, or is expiation required, or is repentance sufficient?

Swearing does not invalidate the fast according to the general consensus of scholars. Therefore, your fast is valid and does not require making up. However, your action has gone against the guidance of the Prophet, peace and blessings be upon him, who instructed that fasting is a shield and that a fasting person should not utter obscenities or act ignorantly. If someone verbally abuses him, he should say: "I am fasting." Furthermore, you have disobeyed Allah by insulting the father of this young man, for it is not permissible to insult someone who has not wronged you. Allah Almighty says: "And no bearer of burdens will bear the burden of another." Therefore, you must repent to Allah and pray for the father of this young man.