What is the ruling on someone who apostatizes from Islam then returns to it? Is he required to make up for the missed pillars of Islam, or is repentance and a fresh start sufficient for him?

The correct view among the scholars is that if an apostate repents and returns to Islam, he does not need to redo the acts of worship he performed before his apostasy. This is because Allah Almighty made it a condition for deeds to be nullified by apostasy that one dies upon it, as stated in His words: (And whoever of you apostatizes from his religion and dies while he is a disbeliever - then those are they whose deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire; they will abide therein eternally.) Al-Baqarah/217. The implication of this verse is that whoever repents before death, his deeds performed before apostasy remain valid and sufficient.