Contact https://www.oag.state.tx.us/cs/. They will have access to collection/enforcement measures if it's doable.
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You can file a motion to enforce child support through the Attorney General's local office. They will confirm his child support arrearage, and give him the chance ( or 2 or 3) to make his payments more regularly & consistently, and if he doesn't, they will invite him to spend a few months in jail. At least he does have a relationship with his kids. That's a good thing.
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