Reference: Fataawa Ramadhaan – Volume 2, Page 664-665, Fatwa No.671
Fataawa as-Siyaam – Page 123-124
Question: A Muslim apostated and then during the day on the 15th of Ramadhaan he repented [and re-entered the fold of Islaam]. Is he required to make up what has passed [of the missed fasts]?
Response: If an apostate repented [and then re-entered the fold of Islaam] during the day [in Ramadhaan], then we ask him to abstain [from all that breaks the fast] for the rest of [the day and assume the fast for the rest of] the month, whilst he is not required to make up what has passed [of the missed fasts].
And this ruling is also applicable to the general disbeliever; And the evidence for that is when the people of Thaqeef entered the fold of Islaam, it was during the ninth year of Ramadhaan and they were the last of the people of Hijaaz to accept Islaam. So when they accepted Islaam the Prophet ﷺ did not command them to make up [what had passed them of the missed fasts] from the beginning of the month [of Ramadhaan], rather he asked them to abstain [from all that breaks the fast] for the rest of the month.
And Allaah [alone] has the Complete knowledge [of all affairs].