Is it permissible for a mother to disinherit her son, claiming she did not receive her inheritance from his deceased father, or that his paternal uncles withheld it from him?

You have no share in the inheritance of your mother's second husband. Your mother is not allowed to give a portion of her wealth as a gift to your brothers while depriving you, because justice among children in giving is obligatory. Your mother cannot disinherit you after her death, even if she makes a will to that effect; rather, her estate will be distributed according to Islamic law. It is not appropriate to discuss your mother's inheritance with her, as if you are wishing for her death. A mother is not allowed to wrong her child, but the mother's right is greater; therefore, it is not permissible for a child to disobey her. The mother has the right to claim her share in her husband's inheritance, and it is preferable to reconcile or resort to the courts or scholars.