nelson/olson

Looking for information on my gf Lewis Nelson b Aug 1845 in Christina, came to the US in July 1856. He enlisted in the Civil War in Dec of 1861 using the name Olson as John Olson signed for him. Lived in Decorah, IA all his life. John Olson was either a uncle or brother-in-law. I would like to find information for Lewis parents.