He's defnitely rapping as Jimmy. Watch the video: it goes forwards to a time when Jimmy has blown up and is performing. It's an allegorical verse; aimed at mimorring Em's own rise to fame. Note the references to fatherhood in the second verse too.
It's easy to see why you might think it is from Em's point, but with the video, the film and the lyrics in mind, Jimmy's character relates a lot more tangibly.