Is continuing to fast voluntarily on Mondays and Thursdays considered an act of disobedience, given that the mother objects to it fearing harm to her son's health?

Islam has given great importance to parents and has made showing them kindness one of the best acts of devotion. Filial piety is the best of deeds after prayer, and it takes precedence over Jihad because it is an individual obligation (fard ‘ayn). Therefore, if a voluntary fast conflicts with showing kindness to parents, the kindness to parents takes precedence, especially when it causes them hardship on the son. However, they should not be obeyed if they instruct to abandon the regular Sunnah prayers. The correct approach might be to differentiate between what causes them hardship, such as the effort involved in fasting, and what does not cause hardship, such as regular supererogatory prayers. They should be obeyed in the former but not in the latter. The reward for fasting and the reward for filial piety can be combined by abandoning a voluntary fast if it causes them hardship. In this case, one earns the reward for fasting due to the intention to fast, and the reward for filial piety by obeying their command. If the son can combine fasting with the parents' satisfaction, that is preferable, either by convincing them that fasting usually does no harm, or by concealing the fast from them if it does not lead to neglecting something they need from him.