Could he perhaps have been a grandchild?  Archival documents on Alexander’s male descendents still left me empty-handed.
Then Dr David de Klerk from Worcester recently sent me a web link to the data at Anglo-Boer War Museum in Bloemfontein, where Alexander Oscar’s and Robert Gert’s date of death is given as 3 March 1902.  Which date is correct?
And just this week (9 May 2013) a friend and researcher in Waterval-Boven, Piet Schoeman, mentioned that the “joiners” - or “National Scouts” as they were called by the British - often shot dead Boer combatants.  Could this have happened to Alexander and Robert?  According to a source that Piet consulted (Military History Journal Vol. 1, 1998), Alexander was an assistant-field cornet who managed a heliograph station at Elandskloof.
Dr De Klerk also provided another link to the University of Cape Town’s data on the concentration camps. And Aha, there I find the entry under Aletta Petronella BASCH, born Stoltz - at the time in the Barberton camp - that “Robert Daniel, aged 15” was with his father Alexander Oscar on commando.
Whilst the second name and age differ from the other sources, I feel the probability is strong that we are dealing with the same individual:  Robert BASCH who fought in the Anglo-Boer War and was killed.
At least his name will live on at Bergendal.