I did some quick (relatively) searching, and I did not pin point his death yet, but what did find, was a William J Plummer (b1857) in MA, in 1910 Census. Listed as widowed, a lodger and at the same place as George C Plummer (b1887):
United States Census, 1910," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M2VG-RGW : 29 October 2015), William J Plummer in household of Charles E Forbes, Somerville Ward 1, Middlesex, Massachusetts, United States; citing enumeration district (ED) ED 987, sheet 8B, NARA microfilm publication T624 (Washington, D.C.: National Archives and Records Administration, n.d.); FHL microfilm 1,374,617.
Back-tracking a bit, we find George Coolridge Plummer born 11 jan 1887, to William J Plummer and Lucy m:
Massachusetts Births, 1841-1915", database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FX6W-6F3 : 1 March 2016), William J. Plummer in entry for George Coolidge Plummer, 1887.
More digging shows Lucy may have died before 1896, as William Sr. is found in 1900 with George, and William Jr., and a 2nd wife (married 4 years), Eliza, and step-son, Charles Lawrence:
United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M9YX-FY4 : 6 March 2015), William J Plummer, Precinct 7 Boston city Ward 14, Suffolk, Massachusetts, United States; citing sheet 7A, family 133, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, n.d.); FHL microfilm 1,240,682.
Looking at 1920, I do not see William Sr. , but George shows up in MA, and William Jr.
So lacking any other records online (free), I would focus on the 1900-1909 window for death. We are also assuming, he died in MA. Census shows George was a railroad worker, and William Sr appears to be a machinists (possibly for a railroad as well). And railroad men can turn up anywhere.
May not answer the question, but maybe leads you down the path. Good luck.
