Would gently advising one's father to perform the obligatory prayers, supererogatory prayers, and supplications be considered an act of disobedience?

Advising a father to increase his acts of obedience and closeness to Allah is not disobedience; rather, it is among the best forms of filial piety, provided that the advice is given with politeness and gentleness. If the father does not accept the advice and is hurt by it, the son must stop advising him and not burden him, as Imam Ahmad stipulated. There are other means of advising a father, including being kind to him, honoring him, and supplicating for him in his absence.