He would have been 18 in 1917 when the US declared war. There could have been a lot of reasons he didn’t go while his older brothers did. Back then, they used to reject you if you had ‘flat feet’. Not every man of military age served in that or any other war. Thankfully, we have never been in that position.
Read somewhere that the US Army was alarmed at the medical rejection rate for WW1 which was somewhere around 20-25%. The big problem was the effects of malnutrition. It wasn't much 'better' for WW2.
My father had pneumonia real bad as a kid. He enlisted in the Air Force in the late '50's and served 6 years. He could easily have flunked a physical due to residual lung scarring. His childhood was at the very beginning of the antibiotic era & Polio was a big problem, too.