Carol,
I recommend that you order his pension application file from the National Archives. It should have specifics on his death and where he is buried. See the following link. Depending on when he died, he may also have a death certificate filed in Oklahoma. A death certificate will identify the cemetery he is buried in.
If you can ID his burial location, then you can work on ordering a government headstone for him.

https://eservices.archives.gov/orderonline/start.swe?SWECmd=GotoView&SWEPostnRowId=1-29XS&SWEView=GPEA+Product+Catalog+Category+Detail+View+FFO&SWEHo=eservices.archives.gov&SWEPostnApplet=GPEA+Product+Catalog+Category+Form+Applet+FFO