Due to her location in Paulding County, GA and in St. Clair County, AL, I suspect his mother is Patience "Patient" Franklin. DNA testing of descendants of Patience's son William Franklin who appears on the 1850 Paulding County, GA census with her could confirm the connection, if he has descendants.
He served in Co C Hardie's Battalion Cavalry Reserves, enlisting on 13 Jun 1864 at Ashville, St. Clair County, AL
Merrell is buried in Liberty Cemetery, Odenville, St. Clair County, AL.
It may be possible to confirm family relationships with Merrell by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
Franklin-6959 and Franklin-1104 appear to represent the same person because: Clear duplicate. It appears Robert accidentally added much of this family rather than linking to them, now we have a lot of merging and clean up to do. Will start with Merrell, project participant line.