This is not really a military law question, which generally deals with military charges. He needs to contact an attorney in the city where the warrant was issued and have them handle it. His military status is not the relevant issue, as these are civilian charges.
Actually. He would be best advised to talk with one of the Legal Assistance officer on his base - it is free.
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Most likely he has a bench warrant with the applicable county. I think a legal assistance officer on his base could make a call to the county police to work out the confusion.
This may affect his military career--he should see a JAG first, may need to hire a civilian attorney afterward.
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His first step should be to see a legal assistance officer. He may have some relief under the Servicemember Civil Relief Act if a default judgment was entered. As far as the warrant, he will need to have the warrant quashed. A local attorney in the area will have to go into court to do that. The court date for the child support will not likely occur within the two weeks. He should have more freedom once he gets out of AIT to attend the court date. There a legal assistance attorneys in practically every duty station. He needs to seek them out. As my colleague indicated, they are absolutely free and there to serve servicemembers.