He is in several census listings. Didn't find his marriage record or that of his parents.
There is more on familysearch relating to his son Robert.
US Census, 1910 Coldbrook, Warren, IL
https://familysearch.org/pal:/MM9.1.1/MKFQ-B7C
self William Slater M 27y IL
wife Kate Slater F 23y IL
son William Slater M 4mos IL
U S Census, 1920 Muscatine, IA
https://familysearch.org/pal:/MM9.1.1/M81B-WT3
self William Slater M 36y IL/IRE/IRE
wife Catharine Slater F 32y IL/NY/IL
son William Slater M 10y IL
U S Census, 1930 Sweetland, Muscatine, IA
https://familysearch.org/pal:/MM9.1.1/XMVZ-MQV
head William Slater M 49 IL/IRE/IRE
wife Catherine Slater F 42 IL/NY/IL
son William Slater M 20 IL
United States, World War II Army Enlistment Records, 1938-1946
https://familysearch.org/pal:/MM9.1.1/KMFP-ZL4
name: William H Jr Slater
Military Service
date: 26 Jun 1943 Cp Dodge, Herrold, Iowa, United States
White citizen birth 1910 IL
education: 2 years of high school
civilian occupation: Stock clerks
marital status: Married
rank: Private
army branch: No branch assignment
army component: Selectees (Enlisted Men)
serial number: 37673988
nara publication title: Electronic Army Serial Number Merged File, ca. 1938-1946
arc identifier: 1263923
card, box and reel number: 12476.13
U S Census, 1940 Seventy-Six Township, Muscatine, IA
https://familysearch.org/pal:/MM9.1.1/KMBS-LQW
head William Slater M 56 IL
wife Kathryn Slater F 52 IL/NY/IL
son William Slater M 30 IL
daughter-in-law Agnes Slater F 26 IA
grandson Robert Slater M 2 IA
grandson Donald Slater M 0 IA