I can only find him listed as William G Allen on censuses, marriage records, etc so not found confirmation of his middle name yet. Unfortunately the marriage record doesn't give information about his parents.
There is a Huldah T Hester, age 64, with William and Rachel and family on the 1860 census, who I assumed was Rachel's mother, but from Googling around, I have found various old message board posts suggesting that she was William's mother, and that she was the Hulda Drew who married Stephen Hester in 1851. If so, then she is down as Huldah T Drew, in Laurens County, on the 1840 census.
This does seem to fit with this 1850 census entry for Laurens County (found the link from a public ancestry tree which has a lot of records etc attached):
http://interactive.ancestry.co.uk/80...47/edit/record
H T Drew 54 F
W G Allen 33 M Farmer
John S Drew 17 M
Georgina Drew 16 F
Rachael Drew 25 F
Susan Drew 6 F
Franklin Drew 5 M
Julian Drew 3 F
William Drew 1 M
S B Hester 66 M
Just wondering whether some of the surnames are incorrect? They are dittoed on the image. They seem to fit with the children who are down as Allen on the 1860 census? Of course it could be the 1860 census which is wrong. Are the older ones maybe Hulda's stepchildren, children of a Mr Drew who she was married to after the death of William's father? And is this the same Rachel as the Rachel who married William?