I'd have time talk to the therapist about it - it would let the therapist know, and your son can get the proper language that way to know what to say. Perhaps the therapist will invite dad and stepmom to an appointment to set some boundaries and let them know what helps and what hurts the therapeutic process. It would also help if the dad would tell the stepmom to step back.
But if you create a tension between you and stepmom, it builds up more. Your son may need to be in therapy because of all these questions and all this feeling like he's on display or constantly having to report. If the dad is too uninvolved as a father, he may have married a more dominant sort of woman - and she may feel if she doesn't get involved, nothing gets done. Maybe she knows he's weak as a father?
Get the therapist involved and develop a plan.