If there is a visitation order in place, the mother moving without telling your husband where probably violates it in two ways, as most orders require the parents to keep the other informed as to residence, and she is denying him visitation. This likely take going to court, but it may be the only way.
If the mother has not informed CSO where she is, you may ask that they do not send her any support payments until she advises where she is living and her residency is confirmed.
As you have sent a letter to CSO, you may want to consider sending one to the judge who oversees the case, or copying him on the one sent to CSO. As with chicken soup, it couldn't hurt.
Finally, you may hire a private investgator to try to locate her. If you do, make sure that your give him/her a budget so that the expense does not get out of hand.
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