Is it permissible for a mother to refuse her son's marriage on the pretext of his financial insufficiency to form a family, and to encourage him to engage in illicit relationships?

If you are able to marry and fear falling into the forbidden, then you must marry and not obey your mother in her refusal, while trying to convince her. It is not permissible to encourage a mother to commit haraam, and you must advise her and clarify the unlawfulness of that. If you are unable to marry, then you must fast, for it is a shield for you, as the Prophet, peace and blessings be upon him, said: "O young people, whoever among you can afford to marry, let him do so, for it helps to lower the gaze and guard modesty. And whoever is not able to do so, let him fast, for it will be a shield for him."