I am very much honorable to a mathematician who is a non-Muslim. Recently, he died.
Can I say إِنَّا لِلّهِ وَإِنَّـا إِلَيْهِ?
Can I pray for him to Allah (سبحانه و تعالى) like we pray when a Muslim dies?
According to a fatwa of scholars, you can say "إِنَّا لِلّهِ وَإِنَّـا إِلَيْهِ ?" because it's not a prayer. It is said, on every occasion, when we are faced with any trouble or calamity. But just keep it to "إِنَّا لِلّهِ وَإِنَّـا إِلَيْهِ ?", not more. Or you cannot pray for him or end any blessing to him, as it's prohibited in Islam.
Rest, you can get details on Can one say inna lillahe…at the death of a disbeliever?.