That would explain why I couldn't find him in 1880...bet you're right. Nothing promising turns up in IL, so prob a mistake. It's amazing how many people don't know where their parents were born or even their mother's maiden name.
I found the 1860 census that shows the Fred/Margaret parents mentioned in another post. With all the uncertainty, do you feel it's definitely the wrong one? With names like Fred Smith and Margaret Kingel, I was already thinking of a German background. The age for Jackson in that 1860 lines up better with a birth year of 1854 (like the 1900 census) than 1859.
You've probably checked - there are records on familysesarch for some of Jackson and Hattie's children -- errors/inconsistencies there too, as if the family just didn't talk about names and dates much.
Sorry I can't help more -- good luck!