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

                If a person left his parents as kaafirs, and he does not know whether they became Muslim or not, is it permissible for him to pray for them?
Praise be to Allah.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]