The privilege belongs to the client, not to the attorney and it can only be waived by the client, not the attorney unless told to do so by the client.
So he probably knows everything that you think that he knows but whether he shows up voluntarily or is subpoenaed, his response to every question, other than his name will have to be:
"My client has instructed me not to answer on the basis of the attorney-client privilege."
He does not have to give the committee even the name of his client.