Praise be to Allah.
If his father is known to have been a good and righteous Muslim, he should not believe anything that he is told to the contrary by those who are not of sound character. It is Sunnah for him to make du’aa’ for him and to give charity on his behalf, unless he knows for sure that his father died in shirk. That knowledge would come about if he has the testimony of two or more sound witnesses who can testify that they saw him offering a sacrifice to someone other than Allaah, such as the occupant of a grave, etc., or heard him calling upon anyone other than Allaah. If that happens, then he should refrain from making du’aa’ for him, and his case rests with Allaah, because the Prophet (peace and blessings of Allaah be upon him) asked his Lord for permission to pray for forgiveness for his mother, and Allaah did not give him permission, although she had died during the Jaahiliyyah as a follower of the religion of the kuffaar. Then he asked his Lord for permission to visit her grave, and He gave him permission. That indicates that whoever dies in shirk, even if he is ignorant, we cannot make du’aa’ for him, or pray for forgiveness for him, or give charity on his behalf, or perform Hajj on his behalf. As for those who die in places which the call of Allaah has not reached, their case rests with Allaah.
The correct scholarly view is that they will be tested on the Day of Resurrection; if they obey, they will go to Paradise and if they disobey they will go to Hell, because of the saheeh ahaadeeth which have been narrated describing that.