I am sorry to learn of this situation.
Your scenario is a little complicated, but focusing on the issue of your former employee sending out a falsified letter, I can assist you with that.
This is an issue of "ostensible authority" in which the third party (the recipient) receives a letter which appears to have the authority and approval of the purported sender (you). This ostensible authority is created by the position of the individual. The third party is entitled to rely on that letter, unless and until they have actual information that the individual does not have the authority to send the letter.
In your case, you can rectify this by sending a letter to third parties simply notifying them that this individual no longer works for your company as of xx/xx/xxxx date and that you would appreciate notification of any correspondence that they receive from him on your letterhead.
Most large businesses deal with employee issues (including issues surrounding employee separation), and if you are still supplying the same products that were agreed upon in the first place (nothing has changed other than this individual causing undue confusion), they are unlikely to change their position.
You have a cause of action for "intentional interference with economic advantage and/or contract" against your former employee, as well as fraud, but unless he is fairly well to do, it may not be worth the effort to pursue the matter, you may however, want to consider having legal counsel send him a letter notifying him to "cease and desist" any further action - simply enough to stop further such actions.