Jimmy wasn't a rat. At least to my knowledge, He got picked up along with Pussy, he had a gun rap. But he would've gotten less time, Pussy was wrapped up in a heroine charge, and he couldn't take the time because of his kids. I forget the cops name but the cop told Tony it was Pussy the first time. Tony didn't believe it, he knew Jimmy got arrested along with Pussy and asked the cop was he sure, then the cop began to question himself because all he knew was that the rat was a heavyset Italian with black hair. The cop said he'll get the files and find out if it was Pussy or Jimmy. Then he got arrested in the cathouse along with Raymond, and a couple of hookers. Before the cop could get the file, it was suspected he had ties to Tony's crew due to the arrest, thus losing his job and he took a header off the bridge, killing himself.

Tony never really found out who it was. But he assumed it was Jimmy due to his trust in Pussy and Jimmy asking questions about the Espanade (I think it was). The producers never really implied Jimmy was a rat, other than his snooping, but that was him being greedy and wanting a bigger cut of whatever operation they were working out, as opposed to wearing a wire. Pussy had been the rat all along.