As for WWI card, if he enlisted before registration date, (Jun 5th 1917) for his birth year then he was not required to file one.
Have you looked at the John Welch age 22 b. TX, parents Texas, is a Guard at the Ramsey State (Prison) Farm in census of 1910 TX Brazoria County?
Also if you are just relying on Ancestry's index for WWI Draf Registration Cards, I feel that not all cards have been scanned or some may be missing from index. To be sure he didn't have a card you should go though the original NARA source (at a library or LDS Family History center). I have been using Ancestry ever since it started supporting Rootsweb. I have found not all results are 100% accurate.