All you can do is your part and have your CPA send the IRS a letter that the ex refuses to amend the return. The IRS will go after him then. You no longer have control over him and this is really not an issue for the court to decide. Maybe you should have him google articles about how Wesley Snipes liked serving his prison term for jerking around the IRS. They are the only profession other than used car salesmen that have a lower public confidence than attorneys.
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You don't need his cooperation to file an amendment, then he can deal with the IRS. Tell CPA to do what he can without the ex.
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