One of your son's Facebook friends could have provided the Facebook information to the principal, which in turn led to the actions taken against your son by the school.
If the Facebook information was provided to the principal by one of your son's Facebook friends (the student perhaps concerned about the publication of his/her own grade information online), the student's identity will probably remain confidential and the reporting of the information should be covered by privilege (an immunity from civil action granted those reporting the potential illegal acts or actions of others).
In other words, should the above apply, the student-reporter's identity would be protected from disclosure, absent a court order which demands release of the identity of the person who made the report to the school. A court will ordinarily not issue such an order unless a lawsuit has been filed with evidence showing the need for the release of this identity.
Schools, by the way, often look at what is published online about the school and the students that attend the school. A lot of this is now done in an attempt to prevent any Columbine-like attacks by catching early warnings of a plan made in a student's postings, but this internet surveillance is also done to protect the school and the teachers in the school from unwarranted, perhaps defamatory, statements made by students about the school or teachers in the school.
Legal actions taken against students for their postings online are not exactly common, but they could hardly be considered rare, either. Teachers have sued students in the past. Kids have gotten into a whole host of trouble for what they have posted online.
Whatever the facts are in your son's case, it appears from what you have posted here that the suspension was a legitimate one. Although I doubt that any legal action will be taken against your son, that doubt is based entirely on the odds in general of a suit being taken by a school against a student. For the chances of your son's particular school going beyond just a suspension in this matter would take a personal review of all of the facts by an attorney in your area.
Good luck.