What is the ruling on the action of a mother who refuses her only son, who is financially comfortable, to marry, despite his need for marriage to protect his chastity, under the pretext that she needs his service, and is it permissible for the son to marry despite her refusal?

You are not obligated to obey your mother if she forbids you from marrying, for refraining from marriage is not an act of righteousness towards her. If you are capable of bearing the expenses of marriage and fear hardship for yourself or falling into sin, then marriage is obligatory for you, and it is not permissible for you to obey your mother in abandoning it.