as salam alaikum
It has been reported from the Prophet, peace be upon him and his family, the following words:
"When a son of Adam dies, his deeds ends except for three: sadaqah jariyah, beneficial knowledge and a pious child who prays for him".
This hadith is found in "Sahih Muslim", "Bihar al-Anwar", "Muniyat al-Murid" and other sources.
Therefore one of the best acts a son can do for his deceased parents is to pray and ask forgiveness for them.
With prayers for your success.