Can you write a "script" and tell your therapist the same things you are saying here; tell him to "push" you?

Don't have regular appointments? How does that work?

I would start writing, keeping a "journal" or blog or whatever you are comfortable with and getting some "ideas" of things to talk about. I would write letters to the therapist and to get the words out of yourself and down on paper.

I would look at your situation too and see if that is not drying up your words. I wouldn't have words either if I didn't feel "safe"? I had problems with not having words and my therapist had to "teach" me to find words by various means but I don't know that your therapist is doing much? What is this particular therapist doing for you, do you know?