First of all, in virtually every jurisdiction in America, he is past the time enabled for an appeal. Nearly all states have a statute of limitations which only permits the filing of an appeal for 30 days after the issuance of the final decree.
If he can show an actual basis to set aside the decree, such as fraud under civil rule 12 B (6), the burden falls on him to show a legal basis to set aside the decree and the settlement. Under that circumstance, if the decree is set aside, there is no liability – each of you start over. New action--so no damages.
In terms of "fraud" or "forgery", you were witness to the signing of the decree. He may have even signed it in court. In any event, there was probably a notary or someone else who can verify the signature on the record. Awfully hard to prove fraud. And in all fairness, I'm sure that the Court served him with a notice of the final hearing, and either he appeared or he have the opportunity to appear at the hearing. If he appeared, he didn't object. If he didn't appear, he had the right to appear and he decided not to appear.
Of course, this is without looking at the specifics of your case, but normally, Courts tend to enforce their existing agreements, and motions of this nature only prevail in a very small number of cases, in which the moving party has to provide a compelling level of proof. Good luck.
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There's not a lot you can do about his threats to file suit until he actually sues. In that case, you should talk to one of us local family attorneys about the suit. If he is frequently bothering you then one thing you can consider is filing a harassment report with the Grapevine police. That might scare him into leaving you alone. It will create a paper trail of his harassing behavior which may be useful in proving his forgery action is just intended to harass you. It would also not be a bad idea to keep any emails, social media messages/posts, texts and mail you receive from him about the alleged forgery or any other threats against you.
If you really wanted it addressed now you could have one of us local attorneys send him a cease and desist letter telling him to leave you alone and that should he pursue his forgery allegations it will not turn out well for him. It would be a fairly low cost option but I suspect he would not be scared off by the letter.
It's doubtful that he could find a lawyer willing to pursue his case. It's also doubtful that he could present a legitimate case at this point unless there were serious procedural defects in the divorce case.