She might have some insight into this that isn't readily apparent. Still, I would think she would discuss that with you. Perhaps too, your son has given her the impression that his height issue is out of boundaries to talk about. Nevertheless, what a struggle - especially as we know our children best. You must be pulling your hair out.

My 15 year old niece is struggling in therapy. It is even a bigger struggle for her parents as they see no improvement. She just doesn't want to talk. I'm not sure what the T is doing/able to do in such a case.

I am ticked at your story incidentally as, at that age, the doctor patient relationship also includes parents. Are you able to use one of the sessions to take the time to have a full discussion with the T? You have the right I believe to sit in on a session too.