First step - add the sources for his profile - show a time line
add the census records with all of the connections
open the profile so that others can add material.
You have a good start. You did not mention death certificate or marriage certificate, do either exist?
Do you have anything that would suggest a relative? Brother or sister aunt or uncle? Anyone living with the family in the census for 1860, 1870 1880 and 1900 that you are not certain of? Where was Aurelia in 1910? Have you traced all children from all of the census records? Sometimes they go to live with grandparents.
In 1850 Douglas was at school, does the school still exist? any records?
Is there an obituary for his death? Does the church he was at have anything, or the local historial society? Is there an obituary for Aurelia? What burial information is available?
None of this is a guarante, just most of the next possible steps. You may have done most of this, but worth double checking and being sure you did not miss even the smallest item.
What have you learned about the Plummer name?
While it is generally not a good idea to just make wild guesses - in the 1850 census I only found one record for a male of the right age to be his father b.VA living in SC with a wife b. SC that might be the mother, Charles Harrison b. VA 1795 living in Columbia SC. A small amout of time looking at this family might be worthwhile. Of course his parents might have been deceased by 1850 or ???
Best of luck.