You are an anonymous inquirer - so I don't know your age: whether it's 15 or 85; however, for purposes of searching, I'm assuming you're about 40.
I'm assuming each genearation is 25 years apart: so your're great-grandfather Jack Jennings was board about 115 years ago: or at the turn of the century.
There is a John Jennings: born 23 Aug 1893 in Wilson County, TN, USA that has a draft registration for WW1 during 1917-1918 found on familysearch.org. He was single at the time.
https://familysearch.org/pal:/MM9.1.1/KZ6X-5BZ
More information may reveal better results.
Good luck!
