Can you have the picture enlarged to where you can seehis left side collar button? The Army issued that buttons with insignia of unit. ie. Transportation, Quartermaster, Medical, Signal etc. Also any patches visible on sleeve.
Check to see if he joined any Veterans groups in he cities he lived in. His membership information (if they keep it that long) may have some information.
Have you contacted the cemetery to see if his headstone has any mention or the cemetery records themselves, if he had a Military Honors.
Contact the MN Historical Society for WWI miliatry information. Also the State Archives as they may have had benefits for the Veterans. Also look at historical newspaper websites for local ones in the counties he lived in as they may have interviewed him on Veterans Day maybe a 10th, 25th, 40th year milestone since Novenber 11th 1918.