Desperately seeking Thorvald (Thorwald) Carlson

Please help me solve the mystery of my great grandfather Thorvald (Thorwald Carlson). According to records he arrived in New York on April 18, 1891 aboard the Island from Norway. We have my great grandparents engagement and wedding announcements and they list his "hometown" as Rockford, IL. He married Libbie Nelson from Michigan City, Indiana, (her parents were from Sweden), and they had four children: Norman Carlson, Harold Carlson, Elwood Carlson (my grandfather) and Ellen Carlson. Approximate d/o/b for Thor would be 1870 in Norway. We also have the "Dear Jane" letter that he left Libbie around 1906 telling her that he would be gone for some time and hoped the Bond money would help. There is absolutely nothing else ever again about Thor. I do know that he served in the Spanish American War as a musician in the Indiana military branch. Anything you may know or suggestions would be great. I have exhausted pretty much everything that I know how to do or that I may understand. This was a very painful event that trickled down through Thor's children. I wanted so badly to solve the mystery for my father, John Harry Carlson, son of Elwood and Margaret Forrester Carlson, before he died. I wasn't able to. He and I shared our passion for our Norwegian heritage. Thank you.