He is consistently listed as single, 1918-1930, living with Isabella/Charles. Then in 1940 is with Virginia. Eva lists herself as divorced on the 1930-40 census. in 1920 she is living with her parents. Didn't see any marriage records for either.
U S Census, 1940 Ward 11, Hartford Town, Hartford, CT
Citing this Record: "United States Census, 1940," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KWMR-9J8
Head Eva Mercer F 40 1910 CT Divorced
Son Robert Mercer M 18 CT
U S Census, 1930 Hartford, Hartford, CT
Citing this Record: "United States Census, 1930," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/XMPV-6MB
Head C H Leon Crawford M 36 CT
Wife Flora Crawford F 35 Eng
Son Richard Crawford M 4 CT
Sister-in-law Eva Mercer F 30 CT Divorced
Nephew Robert Mercer M 8 CT
U S Census, 1930 Hartford, Hartford, CT
Citing this Record: "United States Census, 1930," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/XMPV-FBZ
Head Robert A Mercer M 29 CT Single
Mother Isabella B Mercer F 74 Eng Widow
World War I Draft Registration
Citing this Record: "United States World War I Draft Registration Cards, 1917-1918," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KZFC-WTP
Name: Robert Allison Mercer
Residence 75 Julius, Hartford, Hartford
Draft Registration Sept. 12, 1918 Hartford City no 1, CT
White Male High School Student
Birth Date: 11 Apr 1900
Nearest Relative: Mother, Isabella Mercer, 75 Julius, Hartford