Again, to be sure, you will need to obtain his civil birth and marriage records as well as any church baptismal and marriage records.
It does look like the John listed below him in the 1930 census is his father though since the birth places of his parents match John and Mabel's birth places.
Mabel Klein died in 1945:https://familysearch.org/pal:/MM9.1.1/N3W2-MQQ
John Klein dies in 1940:https://familysearch.org/pal:/MM9.1.1/NQMJ-WSC
I would definitely order obituaries to see what other family members can be identified
It appears that Mabel - may be Emma Mabel?
Walter Roy Klein appears to be born Oct 29 1888 in Minneapolis, Minnesota.
He lists Henry Klein as his nearest relative on his WWI card and Henry lives in Janesville, WI.
In his WWII papers he lists Mable Klein of Rockford, IL. Henry could possibly be a brother of
the father, John Klein, because in 1880 there was a Jacob Klein who was married to a Bettie Kuhn/Cohn and living in Desplanes, Cook Co, IL with John (1868), Henry (1871), Herman (1875) and Louis (1876).
A John Klein and Emma Parker were married in Cook Co, IL on Feb 4, 1888.
Then a John William Klein and a Emma Rose Parker were married in Racine, WI in 1896
and this Emma is from Hartford, Conn too! Not sure if this is one and the same?https://familysearch.org/pal:/MM9.1.1/XRKV-MD5
Another son, Henry Klein, was born in Chicago in 1905 and died in Minnesota:https://familysearch.org/pal:/MM9.1.1/XPD9-DL4
Another child was born in 1892:https://familysearch.org/pal:/MM9.1.1/FDMD-1YS
In the 1900 the family is living in Elgin, Kane Co, IL
What is the family lore or stories that were told about this family? What other
info or insight can you provide? At this point, definitely follow up on the previous
leads and the basic documents.