I have just remembered a scene from a few episodes back when PJ was in Bertram's office with some of the other RJ suspects, PJ said he was getting closer (or along them lines). Once PJ leaves, Reede Smith asks Bertram just how much PJ knows, to which Bertram say's "that is the question" and he say's it in quiet a creepy way, not in a way that he is just frustrated because Jane is being secretive.

He has been in it long enough for Hellar to have a "look all this time he was RJ" moments.

The actor who plays Partridge has said himself that he is actually dead, so that would rule him out.