There is no age in Texas that a child can decide which parent he or she would like to live with. However, at age 12, the child is able to inform the judge as to which parent the child would like to live with. The judge may take the child's wishes into consideration, but the judge will not necessarily decide as the child wishes. The judge will only allow the child to live with the parent of the child's choice when it is in the child's best interest to live with the specified parent.
I would also add that your brother should be careful not to put the 12 year old in the middle of this decision. It is common for a child to tell each parent (whomever they are with at the time) that they want to live with him/her. Just because his parents are divorcing that does not place the child in the decisionmaker's seat, nor should it. That is a lot of pressure (often from both sides) for a 12 year old.
During the divorce there may be a social study ordered, or possibly an amicus attorney appointed by the court to investigate the matter. Your brother should discuss all of these matters with his attorney so he can get information specific to his case and his son.