Can he pray for his parents when he does not know what religion they were following when they died?

Shaykh al-Islam Ibn Taymiyah was asked this question
and he replied: 

Since he comes from a kaafir nation, it is not permissible
for him to pray for his parents, unless they had become Muslim, as Allaah
says (interpretation of the meaning): 

“It is not (proper) for the Prophet and those who
believe to ask Allaah’s forgiveness for the Mushrikoon, 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).”

[al-Tawbah 9:113]