Is it permissible to perform recommended acts of worship – such as voluntary fasting – despite a parent's prohibition of them without a valid reason, and would this be considered disobedience?

It appears that the father is forbidding fasting out of compassion. Therefore, scholars believe that it is preferable to obey him and refrain from fasting. As for prayer, there is no valid reason for him to forbid it. Hence, it is obligatory to pray, even without his permission, while attempting to appease him. Ibn Muflih narrated from Ahmad, regarding someone whose parents forbid him from supererogatory fasting, that he disliked for him to fast, and that he should break his fast, and he would have the reward for dutifulness and for fasting if he broke it. As for prayer, he should pray and appease his parents. If he swore an oath to fast, it does not become obligatory, and expiation is required if he does not fast. It is preferable to obey the father in refraining from supererogatory fasting, but if it is possible to convince him of the virtue of fasting without harm, that is better.