Nice old photos.
Starting with the family tree listed in your Ancestry profile, I traced him to Swift County, Minnesota 1900 and his widow and children in Idaho 1910, and found his Find a Grave site that lists him as Niels L. Ottosen born 30 Dec 1870:http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=Ot...
Ancestry also lists his enlistment in 10th Infantry 1893 at Chicago as Niels L. Ottosen age 22 born Copenhagen, and discharge 1896 at Fort Sill, Oklahoma Territory.
In the 1900 census, he listed 1890 immigration year, but I don't find him in passenger lists.
Do you have a birth date or other info for the brother Chris, and the aunt's name?