Jimmy already knows in Nocturne. (Sorry I don't know how to direct link it, but do a title search in the archive.)
Also in Foreshadowing which is on the fanfic boards but not the archive. Search the past 30 days.
Jimmy figures it out in Sex, Lies and Photographs (also on the archive).
There's one on the archive where Jimmy and Lois both get Superman's powers in a lightning strike a la Stupendous Man, but I can't think of the title. Maybe someone else can?
