Courts in Texas are supposed to act in "the best interest of the child" when making decisions regarding custody arrangements. Your chances of modifying the custodial arrangement depends on exactly what the original order says. If y'all are joint managing conservators, your burden is somewhat lighter than if she is the sole managing conservator.
Having said that, if you can show the court that continuing to allow your son to live with his grandmother is detrimental to him, then you at least stand a chance. Another factor to consider is the age of the child - the court has the option of listening to the child's desires once he turns 12. The other factor to consider is what court you are in. If you are in Harris County, you have to deal with the politics of the Family Law Center.
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