Praise be to Allah.
He does not have to make up what came before it, because Allaah says (interpretation of the meaning):
“Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven”
i.e., if they give up their kufr and become Muslim, then Allaah will forgive them their previous sins, whether these were sins of neglecting obligatory duties or doing haraam acts. But he must offer the prayer at the time of which he became Muslim, if there remained enough time to do one rak’ah or more, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever catches up with a rak’ah of the prayer has caught up with the prayer.”
As for the prayers that came before, he does not have to make them up, such as if he became Muslim after ‘Asr, he has to make up ‘Asr, but he does not have to make up Zuhr, because the time for it had ended. End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).