L.R.
I'm an attorney so I can tell you that if you have a "standard" order from Texas, it will say what Erika T. told you it says. It is the school district where the child resides. Just because the child isn't in school yet, doesn't mean that you forego that time you would get at Spring Break. Tell the child's mother to read her order more carefully and let her know when y'all will be there to pick him up. If she still refuses, go to pick him up. That way, if she turns you away at the door, then she is in violation of the court order and the judge can give her a good tongue-lashing for trying to come between your step-son and his father. She's only in violation if you go to get him and she still denies the visit. (I get so tired of mom's like that.)
Good luck!