I know that they were confusing him and Pusy with each other when the revelation came but a lot of things point to him being a rat

Pro's for being a rat
-Asking about stuff that's already been covered
-Seemingly trying to get Tony to say something incriminating
-The way he said 'Oh god' when he was called a rat, although it could easily be fear of him impending doom

Personally I would've liked to have seen more of Jimmy rather than Carlo Gervasi as a character