I am going to that therapist and insisting that he tell me why my son is seeing him.
Please don't make a scene. Does your son know that you're insisting to know?
I tell him that's none of his business.
It isn't right to try and pry into why your child is seeing a therapist. It is his own private life, and even if you are their parent you should still respect that they're going through things and want to do it on their own. If you really want to know then you can ask and sympathize with your son, but you shouldn't ask the therapist as that's a breach of trust.
Ok. I'm planning on being there with my son when he talks to the therapist.
If you want to go along with your son you should ask him first. He may not feel comfortable with you being there. Therapy sessions are supposed to be private, and you need to respect that.