My x husband is trying to take two of our kids to Texas we have joint custody and I have other children including a medically fragile child is he able

Me and my ex husband have four children together I have two and he has two but we both share joint custody of all of them I also have 4 other children and one of them is medically fragile he is trying to take two of our children to Texas to live there we both live in the state of Tennessee as of now he says he is moving before March first because he sold his house if he moves to Texas I won’t be able to see my other two children and they won’t be able to see their brothers and sisters how do I stop him I don’t have a lot of money and I need to do it fast he is trying to move in like three weeks