Is a grandfather, who believes in one God and calls Him "Bog" in Russian, considered a Muslim and a monotheist for whom one can supplicate for mercy and forgiveness if he dies in this state?

A Christian enters Islam by bearing witness that there is no god but Allah, and that Muhammad is the Messenger of Allah, and that Jesus is the servant of Allah and His Messenger. Then, he commits to the rituals of Islam. It is not enough to simply affirm one God, for the jurists have clarified that Islam is not established except by affirming what the disbeliever has denied, such as denying monotheism or the prophethood of Muhammad, peace be be upon him. Based on this, the Christian disbeliever must bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah to all mankind, and disavow every religion that contradicts Islam. This must be done in a manner of origination that includes commitment and submission, not merely informing. If he does not do so, he dies a disbeliever, and it is not permissible to supplicate for his forgiveness.