Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
Answering your question, Dr. Mohammad S. Alrahawan, Associate Professor at the department of Islamic Studies in English, Al-Azhar University, Egypt, states:
Allah stated in the Qur’an, “It is not (proper) for the Prophet and those who believe to ask Allah’s forgiveness for the disbelievers, even though they be of kin, after it has become clear to them that they are the dwellers of the Fire [because they died in a state of disbelief]” (At-Tawbah 9:113)
On commenting on this verse, Imam Al-Qurtubi, an exegete of the Qur’an, said: “Allah has not given permission to the believers to pray for forgiveness for the polytheists.” (Al-Qurtubi, Al-Jami` 3:173)
Similarly, the Prophet has been reported as saying, “I asked my Lord for permission to pray for forgiveness for my mother, and He did not permit me. I asked Him for permission to visit her grave, and He granted me permission.” (Muslim)
Therefore, it is not permissible to pray for your grandfather who died in the state of disbelief.
Allah Almighty knows best.