I am sure there are many closet apostates in positions within congregations and at the society. You may never know, and may be that is a good thing.
If he knows TTATT, then there is a reason why he remains in the closet. It is really not up to you to expose him, and it would really not be fair. If you are one-on-one with him you may bring up a conversation about the blood doctrine. It might be interesting.