On the presumption they were from Nova Scotia originally, I checked the 1861, 1871, 1881 Canadian Census at Library and Archives Canada and also checked Nova Scotia Vital Stats. Because of the common first and last names, there are literally hundreds of possibilities. It would be a daunting task to check them all but the original files can be seen on-line. Both sites are free and primary sources. You would just have to keep on looking until you ran into a file with father David, mother Rebecca and son Robert. That would likely be the one. What were the details of the the source that suggested the parents were from Nova Scotia? Can you pin a locality down within Nova Scotia?