RN, I agree with all the great advice you have gotten on this thread. I am coming late to the party--and have not much to add.
I too would try to keep out of it. You can say you are pleased about the job and leave it at that. I do not think we as parents are equipped to be involved in or evaluate our children's response to drug treatment. I would stay out of it.
I get the sense he is manipulating and triangulating. He is holding onto his power, which got him into this mess to begin with. I believe he is making the therapist responsible. I disagree that it is a matter of clicking with the therapist. She is looking for a response, a change in him that is not forthcoming. It is not about her. It is about him.
If it were me, I would not endorse changes. Because any changing things empowers your son to believe it is the program which holds the responsibility and not him. Your feeling about the original place was good at the beginning. I would go with them and their referrals. Unless something substantial takes away from that confidence.
It sounds like your son thinks he knows best, and can guide his own recovery. Well what can you do about it? You do not have any control over what he thinks or does, really, at this stage of the game. It sounds like he is making enough money to subsidize his choices.
In sum, I would stay positive, praise him about his work choices and leave it up to him. I would not endorse changing programs or therapists. I would encourage him to keep at it.