As far as I know there was only a "bonus" payable in 20 yrs given to Veterans. Google WWI Veteran Bonus
He was the right age for being required to file a Draft Registration Card for the 3rd registration day Sept 12, 1918 Those already enlisted were exempt. So it could be he was in the Federal military or National Guard or it could be his card was lost or too faded to scan. Since I can't find him (or wife) in 1930 census then I can't see what he answered for question of being a "Veteran of US Military or Naval Service". Check with Iowafor National Guard records and WWI Veterans data.
This is what I did find:
Son William Lewis age 13 listed as nephew in paternal Aunt's household in 1930 CO Denver County, Denver G.W. Morgan head, wife Elizabeth age 52 b. PA/parents PA, William F Lewis age 74 father-in-law b. PA, Margaret McNamara age 34 b. WY
Llewellyn Lewis, wife Katherine married 9 yrs with daughter Lillian M. age 8 are in a household below his parents William and Mary J. Lewis in the 1910 Wyoming, Sweetwater Co. Middle Twsp. father and son are working in coal mine.
I also believe the Wm Lewis family is in the 1880 PA Carbon Co Twsp. He is a coal miner.