I am not a historian,but i would like to offer a couple of suggestions.
1) When he died,did he have military honors? If he did,what organization performed them. Was it done through the local American Legion Post or through the Veterans of Foreign Wars Local Post.Contacting these local organizations that he may have been a member of may be helpful as he may have spoken about his service or better yet they may have an application on file that states his service. They may also be ablr to give you some other resources.
2) Contact the U.S.Army Military History Institute located in Carlisle Pa http://www.carlisle.army.mil/ahec/MHI.htmhttp://www.carlisle.army.mil/ahec/MHI.htm to see if they have a unit history of his unit and if so,could they do a lookup for you.