Hi Kinn
I would guess that Cunningham's father was John, brother to George, born around 1785 in Wylam Northumberland. Cunningham called his eldest son John. I also found this on IGI, actual extract from OPRs:
JOHN STEVENSON Marriages: Spouse: EUPHONIA RAE
Marriage: 22 DEC 1815 Rerrick, Kirkcudbright, Scotland
Rerrick is also showing on the 1851 Census as Cunningham's birth place. He also seems to have called your GG grandmother after his mother.
You will get final confirmation if you view Cunningham's death cert on Scotlands People. A free search on SP shows a Cunningham Stevenson, born 1819, died in Kirkcudbright in 1896. His death cert will show who his parents were.
Regards.
Monica