What is the ruling on voluntary fasting as a protection against masturbation, when the mother forbids it due to thinness and her unawareness of the real reason?

Fasting to curb desires is commendable, not obligatory. If a mother prevents her son from fasting out of compassion, he should break his fast gently with her. Desires can be reduced through marriage, supplication (du'a), associating with righteous people, remembering Allah the Almighty, and occupying oneself with what is beneficial.