If the apostate repents, he does not have to make up prayers or fasts that he did not do during the time of apostasy, because repentance erases what came before it.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
Does the apostate have to make up prayers and fasts if he comes back to Islam and repents?
He does not have to make them up; whoever repents, Allah accepts his repentance. If a person does not pray, or he does something that nullifies Islam, then Allah guides him and he repents, then he does not have to make them up.
This is the correct scholarly view, because Islam erases that which came before it, and repentance erases that which came before it.
Allah, may He be glorified and exalted, said: “Say to those who have disbelieved, if they cease (from disbelief) their past will be forgiven” [al-Anfaal 8:38]. Here Allah, may He be glorified and exalted, states that if the disbeliever becomes Muslim, Allah will forgive him everything that he did in the past.
The Prophet (blessings and peace of Allah be upon him) said: “Repentance erases that which came before it, and Islam erases that which came before it.”
End quote from Majmoo‘ Fataawa Ibn Baaz, 29/196
See also the answer to question no. 197247
But if before he apostatized, the apostate had missed some prayers or fasts or zakaah, then he must make them up, according to the majority of scholars.
It says in al-Mawsoo‘ah al-Fiqhiyyah (22/201-202):
If the apostate who has repented owed some prayers or fasts or zakaah that he missed before he apostatized, does he have to make them up? The majority of Hanafi, Shaafa‘i and Hanbali fuqaha’ are of the view that it is obligatory to make them up, because omitting an act of worship is a sin, and sins remain after apostatizing. End quote.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: In the case of the apostate, he is not obliged to make up prayers, zakaah and fasts that he missed during his apostasy, according to the well-known scholarly view, but he has to make up anything that he missed before he apostatized, according to the well-known view.
End quote from Majmoo‘ al-Fataawa, 22/10
See also the answer to question no. 105315
And Allah knows best.