If he apostatized then repented, and he owed prayers, fasts or zakaah before he apostatized, then he must make them up
Question

                There is someone who apostatized, and before he apostatized, he had some prayers and fasts that he had to make up but did not. If he comes back to Islam, does he still have to make them up, or is everything erased and does he become like someone who is new in Islam?
Praise be to Allah.
    Firstly: 

    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 replied:

    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

    Secondly: 

    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.