Is it considered disobedience (ʿuqūq) to take the son despite the mother's refusal, or is that permissible due to the necessity of caring for and looking after the son, especially since the father is elderly and blind?

The jurists are of the opinion that the mother has a greater right to custody of her child after separation, as long as she does not remarry. If she remarries, her right to custody is forfeited and transfers to whoever is next in line for the child's care, such as the maternal grandmother. So, if the mother remarries, the maternal grandmother has a greater right to the child's custody, if she is qualified for it, and it is not permissible to refuse to give the child to her. However, if the maternal grandmother is not qualified for custody, then the mother incurs no sin by not giving the child to her, because obedience to parents is conditioned by what benefits them and causes no harm. Custody then transfers to whoever is next in line for the child's care after her. If the one who has the right to custody agrees for the child to remain with the mother after her remarriage, there is no objection.