I think his parents died when he was quite young. On his WWI draft card, he didn't know the month and day he was born. Also he didn't sign the card but put a mark on it.
William Marion Davis
1918 Residence: Algood, Putnam, TN
Occ: Laborer for L. D. Ford, Cookeville, Putnam, TN
Nearest Relative: Fate Davis, Algood
That's all that I found in Ancestry and FamilySearch.
the Douglas County (MO) Historical & Genealogical Society, Inc., www.rootsweb.ancestry.com/~modougla/HistSoc/HISTSOC.htm
might have more info about him. Successful Searching!