Luther is known as the son of Frederick and Elizabeth because he is found in the 1870 Census in Middleburg with Elizabeth after Frederick has died. His tombstone gives his birth and death dates. He was born in 1863 so his first appearance in Census was 1870, then he is not to be found in 1880 (I don't know why), the 1890 Census was destroyed and he died before the 1900 Census -- so he is somewhat elusive.

I found an obituary which said he was of Centreville, died suddenly at about age 35 and left a widow and two children.