Praise be to Allah.
He does not have to make up the days that came before he became Muslim, because at that time the command to fast was not addressed to him. He was not one of the people who are obliged to fast so he is not obliged to make it up. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him)
Al-Ijaabaat ‘ala As’ilat al-Jaaliyaat, 1/8