Have you told your T that you're furious with him. . . for what? Seeing your son's girlfriend/roommate too? I'm not clear why you have lost faith in him, only why you might be (I think I would be too) angry. If you have not talked to your T, gotten his "side" of what your son has told you, asked him pointed questions about where/how there is no conflict of interest given you can't stand the woman and are paying for your son's therapy, etc. I think I'd see what the story is from your T's side before I did anything else.

You said your son shared, "that he is bring his EX girlfriend... into therapy with him" -- do you know if this has happened yet or if it's what your son intends to do in the future? This could be an idea of your son's that your T doesn't even know about much less condone/agree with? You have to talk to your therapist first, not go with whatever second and thirdhand (your head messing with you) stuff you are told/imagine. Go toe-to-toe with your therapist and get what is actually happening and if it isn't to your liking, say so and stop it! If you are paying, I don't think your T is going to see your son if you don't pay.