chic, in all fairness, I can see that he would feel more trapped if he thought you were going to know his interaction with his own doctor.
Be sure to drive home the point that it is a different doctor of his own who will not trade information, maybe just say that the offices are together but they each treat "different sub specialties" of patients.
you could even go so far as to say your own therapist "specializes in older women" (who want to rip their adult children's head off at the neck for living with them).
I think we tend to resist change if we are afraid....but moving forward you have gained so much.