What can a son do to honor his deceased father who was remiss in some acts of worship, such as prayer, fasting, zakat, and Hajj, knowing that people testified to his good character?

We ask Allah to accept the supplicant's prayer, and to have mercy on his father and all deceased Muslims. You can do much that will benefit your father after his death. If he neglected his fasting, it is permissible for you to fast on his behalf. If he neglected zakat that was obligatory on his wealth, then you must pay the zakat on his behalf from his wealth before dividing the inheritance. As for Hajj, what was performed by someone who performed Hajj on his behalf is sufficient. You can also benefit him by frequently supplicating for him, giving charity on his behalf (ongoing and non-ongoing), and other acts of righteousness and dedicating their reward to him. In Sahih Muslim, the Prophet, peace and blessings be upon him, said: "When a person dies, his deeds come to an end, except for three: ongoing charity, knowledge from which benefit is derived, or a righteous child who prays for him."