Is it permissible to disobey one's mother and fast on Mondays and Thursdays secretly to rid oneself of masturbation, despite her objection to the fasting out of fear for her son's health?

We ask Allah to help you repent. Then, we advise you to consult your mother regarding fasting. If she insists, the principle is that obedience to her is obligatory. You can also seek help from some medications that reduce desire. If these means do not benefit you, and fasting becomes the only way to attain chastity, thus becoming obligatory upon you, then there is no blame upon you if you fast, even if your mother prevents you.