If your grandfather was a heavy machinegunner, he'd have been a member of Company H, which was the weapons company of the regiment's 2d Battalion. Company H provided heavy weapons support for the battalion's three line companies (E, F and G) so he may have been attached to Company E at one time or another.
The regiment was an element of 90th Infantry Division. You can contact the division's veterans association. Perhaps they can put you in touch with someone who remembers him.
90th Infantry Division Association James R. Reed, Secretary-Treasurer 17 Lakeshore Drive Willowbrook, Illinois 60514-2221 (630)789-0204