You should check the state statutes for professinal engineers. In Florida the specific section is in Chapter 471.033.
https://www.flsenate.gov/Laws/Statutes/2017/Chapter471/All
(f) Advertising goods or services in a manner that is fraudulent, false, deceptive, or misleading in form or content.
(g) Engaging in fraud or deceit, negligence, incompetence, or misconduct, in the practice of engineering.
You have many options and they all would need a copy of that letter.
1. Forward the letter to the State Board for their review/comment.
2. I would think a phone call to the old boss would have a quicker effect and might be enough to stop the bleeding.
3. Have your current company send the guy a letter through an attorney. There may be some info in the employee manual that deals with this situation. If he signed the employee manual notice then it could be considered a contract. Or if he signed a contract (non-solicit or non-complete) he may in breach of that contract as well.
4. Have your own attorney send a letter to the guy for libel.
You can take it as a lesson in business that some people are jerks and will do anything to get ahead. Don't stoop to their level, always act professional and your career and life will be better off in the long run.