It appears that the City of Galveston owns and manages several cemeteries, which includes Evergreen. It might be worthwhile to try contacting them, to see if they have any additional information? Perhaps at the very least, they might be able to tell you in which section of the cemetery he might be buried.
http://www.cityofgalveston.org/528/Galveston-CemeteriesA California cemetery in which some of my husband's ancestors were buried has sections which were determined by the decades of the deaths. The oldest deaths are in one section, deaths from the early 1900s are in another, etc. (Of course, relatives who shared a plot may be buried many, many years after the other relative, so this method doesn't always work.)
In our region of California, I suppose a marker could be a generic term for an upright headstone, a flat gravestone or a small marker.
Note: looking at Evergreen Cemetery on FindAGrave, there is a diagram of where the old cemeteries are located - Evergreen appears to be closest to Broadway and 43rd Street. Using an online satellite map, it
appears that there are "missing" grave markers.
Apparently, 67% of the interments have been photographed. I suppose there is still a chance that one day his stone may be photographed, if in fact it still exists.