He needs to file a suit affecting the parent child relationship. With that filing, request a temporary orders hearing and that on hearing, the court grant temporary orders stating neither party can remove the child without permission from Tarrant county (or TX). Once granted, you can have it enforced by the cops if you have to.
As it stands now, there is nothing preventing her from taking the child to another state.
If you file suit, your son will need to consider child support and health care, which will almost certainly be part of the orders. The parents might agree otherwise, but unless they do, the court will order it.
Disclaimer: This answer is intended for educational and informational purposes only and should not be construed as legal advice or forming any attorney-client relationship.
A court order will typically include sections that address when/how/where/etc the child can be moved to and from a different state. As the previous lawyer mentioned, it is important to have a court order because it is your tool for action in the event it is needed. Your son may want to consider a consultation with a lawyer at the very least. The lawyer can outline a plan of action and provide you/him with an estimate on the cost.