Reread your divorce papers. There's probably a section stating that each parent has to notify the other in writing for approval concerning counseling, orthodontia, surgery. Traditional doctor visits don't apply.
Just quote the section.
Good job on not venting on your son. Good job that he felt safe enough to tell you where he was. Who knows, maybe he wanted you to know to get you all riled up so he wouldn't have to go.
The therapist should not be talking about what the kids share with privately. Counseling is supposed to be a safe place. If it's one session the kids know what to say to 'protect' their parent(s) and how to cut all sessions short. "Life is beautiful." Finished, end of story.
Want to see a movie today.. if the weather isn't too ucky? I have the house all to myself!