What is the ruling on a son taking his father to a mosque that contains a grave, and is it permissible to compel him to pray in another mosque?

If the mosque predates the grave, and the grave is not in the direction of the Qibla (definition), and no reprehensible acts occur near it, then prayer in it is permissible, and you must obey your father in taking him to it.

However, if the mosque was built over a grave, or the grave is in the direction of the Qibla, or people visit it for impermissible intercession, then prayer in it is not permissible, and consequently, you are not permitted to take your father to it.

Sheikh Ibn Uthaymeen mentioned that prayer in a mosque built over a grave is invalid and the mosque must be demolished. If the mosque existed prior and a deceased person was buried in it, then if the grave is in the direction of the Qibla, one should not pray in it. But if it is to the right, left, or behind the worshipper, there is no harm, and the grave must be exhumed and the deceased buried with the people.

It is not permissible to compel your father to go to another mosque; rather, you must gently and politely explain the religious ruling to him, without violence or harshness in speech.