Thank you for using our forum. My name is ***** ***** I hope to assist you today.
I am very sorry to learn of this situation.
Unfortunately the problem with this matter is that your claims are not so much against these two individuals and their investigator, but against the entities that were supposed to be protecting the information (so whoever held your son's medical information, and whoever held the confidential settlement documents) - who had the confidential information.
Those are the entities you could pursue.
As far as claims against the PI and his clients, there is a slight possibility that a cause of action could exist, but it would really depend on the details of how the PI went about getting this information (so it would take some more investigation work on your attorney's behalf).
Your divorce attorney is probably wise to refer you out, this matter should be handled by a civil litigation attorney rather than a family law attorney (they are different types of law).
You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).