Hi Barbara! Kust a couple of ideas. First, just because your grandmother didn't go identify him, didn't mean 1)someone else didn't (she, as next of kin, would be the first one called, but friends, family members, employers etc. could do it) or 2)they didn't bury him under his name (they knew enough to contact your grandmother). Check the Texas death certificate images at FamilySearch:
Keep in mind that "John Doe" is not really the preference for unknown death certificate names, at least not in Texas. "Unknown" is usually what is used (and there are ALOT of them).
Other idea: anyone else in his family still alive that might be able to give a hint as to where he was?
How old do you think your father was when your grandmother got the call? 5? 10? 15? That might narrow down a time frame. It's entirely possible that your grandfather came to Texas during an oil boom (lots of folks did). An approximate date would give an idea of which areas of Texas were booming at that time. It's just a place to start.