Are you certain that his parents were actually married? If the mother's name is listed on the son's marriage certificate as "Elizabeth Bruce MS Ramsay", this means that they were married, or the son believed they were married, or he knew they were not but didn't want anyone else to know. If is just says "Elizabeth Ramsay" then they were not married.
Try looking for a Robert Ramsay in the 1851 census and see what comes up.