Jimmy is Superman's best friend. He is named after the movie actor.
Jimmy always informs Clark about the Crime thaking place in Metropolis, usually about Lex Luthor. In the Superman theme song video, he is sitting next to Clark on a bench, presumably waiting for a bus. It could be possible that Jimmy knows Superman's secret identity becuase he always seems to go to Clark, who is actualy Superman, whenever there's crime going on in the city.