You need to have the judge order him to summit his returns in accordance with IRS rules. If the defendant refuses to summit the papers ordered he may be held in contempt of court by the Judge and face a fine and/or jail time. (The Judge could have done this the first time around) This is assuming that the Judge will allow the evidence of the return in the first place. Which from your post it is evident that he will.
The link attached will explain what is or is not required of the IRS.
Take a look at example two of the above link. (towards the very bottom)This will give you an idea as to the detail and complexity of doing this. So let the Judge or Court continue to handle this on your behalf. File a motion again with the Court to release the return or returns following the needed criteria and have the Judge hold him in contempt if he does not. Even yet, the Defendant does have the right to ask the IRS to stop or not allow the release.
For a proper service (subpoena) to be served to the IRS it must conform to their standards as outlined in Sec.(NNN) NNN-NNNN5 of IRB 2005-11. (above link)
Any subpoena/ or request will need to be served to the District Director for the area where the tax information is located. Meaning where the file is returned.
I know this is confusing and frustrating but the above info should better help you understand what exactly the process for doing this is. Hang in there and press the Court to do this again and hold him in contempt if possible.
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