The judge will decide. There is no magic age when children get to decide custody. The best interest of the child is the standard.
The law regarding a child's 'right'* to choose is a matter for each State and jurisdiction. The judge in most States, not the child, makes the decision based on the best interest of the child. Although not a standard by any means, many States have begun to give 'consideration' to a child's declaration of custodial preference when the child reaches the age of twelve or thirteen, sometimes fourteen. There are even cases when children of age 9 are allowed to testify.
The judge is normally given almost unlimited latitude in whether or not she or he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" but the younger the child the less likely for a judge to give the stated preference much weight.
Good luck to you.
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Based on what you have written, the Texas Family Code suggests 12 as the age, but dependent upon case specifics that age may differ. Other than the father telling your son "not right now" what are the reasons why the child can not come live with you? Also, which court issued your current order - as that may have some control over any potential outcomes? If my local law firm can assist you with this legal issue or any other legal matter, please contact us at firstname.lastname@example.org to arrange for an initial consultation.