Looks like from shoulder patch he was in the 3rd Army. If you have a CSI type of photo shop program where you can enlarge and enhance collar button it should have the image of his unit type...Artillery, Quartermaster, Infantry, etc.
About U.S. Third Army "The Third United States Army was first activated as a formation during the First World War on 7 November 1918, at Chaumont, France, when the General Headquarters of the American Expeditionary Forces issued General Order 198 organizing the Third Army and announcing its headquarters staff. On the 15th, Major General Joseph T. Dickman assumed command and issued Third Army General Order No. 1. The third Army consisted of three corps (III, Maj. Gen. John L. Hines; IV, Maj. Gen. Charles Muir; and VII, Maj. Gen. William G. Hahn) and seven divisions."