I found some info at http://www.houseofnames.com/xq/asp.fc/qx/rowbotham-family-cr...
. I'm sure you know that crests don't really belong to the entire family, but are given to an individual. Anyway, the info on the name at the web site might apply. They say the name came from the Lancashire area and means "at the roe-bottom a residence in the depressed ground frequented by deer."
Now that doesn't give you your grandfather's original name, but it might be a clue. Do your family tales include when and/or why he changed the name? Did he always live in England? Did he marry your Grandmother under Rowebatham or Rowe?
Anyone else have any ideas?