Is this him? William Butler Service Info.: PVT US ARMY WORLD WAR I Birth Date: 3 Jul 1888 Death Date: 31 Dec 1958 Service Start Date: 21 Jun 1918 Interment Date: 8 Jan 1959 Cemetery: Beverly National Cemetery Cemetery Address: 916 Bridgeboro Road Bridgeboro Road Beverly, NJ 08010(609) 880-0827 Buried At: Section L Site 4579
Contact the cemetery see if they have records.
Have you gone to the NARA website for help in researching service records? www.archives.gov
Use Google book search to see if the unit has a mention in a book and also search his name.
Most Military units were Federalized State National Guard Units so check State Library Archives.