You are making a serious accusation that could (if true and proven) involve criminal activity. Get yourself an experienced attorney to determine how to address this. This is not a do it yourself project, and your accusation could result in disbarment of a lawyer, jail for a notary, plus a new trial, depending on the details. What you don't do is "sue him for this." I suspect an attorney will look into an extraordinary motion for a new trial, and may recommend a bar complaint as well. Make sure you share all the details and facts. What may work against you however can be your failure to keep up with the case and file appropriate pleadings, so there's no way anyone here can predict the outcome.
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Hire a lawyer first to determine what the best stategy for you in court. At the same time, you might be able to complain to the state bar about his apparent unethical and possibly illegal (forgery) behavior.
You will most definitely need to retain an experienced divorce attorney. There are several things that can be done, depending upon the date of the Final Decree. Do not wait any longer. Please call my office for a phone consultation at no charge (770-986-9364). I can tell you what needs to be done, and the approximate cost.
I am exclusively a family law attorney, practicing primarily in the metro Atlanta, Georgia trial courts. However, I handle appeals from anywhere in Georgia.