Most of the WW-I Divisions that remain or were active during WW-II have Veterans Associations which welcome descendants of bothe wars to be members in one level or another. In our case you need to find out what unit your grandfather served in.
Provide the complete details as you know them in a reply so that someone knowledeable can assist you. Based on where he lived prior to the war could be a significant item in determining which unit he was in since draftees from various regions ofthe country were assigned to different training camps and therefore the specific unit. Full name, date of birth and location, parent's names, where he lived prior to entering the army, as well as where he resided after the war. Ancestry.com (subscription required) has the WW-I draft cards on line as well as census records for 1910, 1930 and 1930
As an example go to www.80thdivision.com to see information on my affiliated organization in which my father served in WW-I.