I'll tell you now that you will have an extremely difficult time finding your connection as the Canadian Rioux spelling is something entirely different back in Brittany, and unfortunately, I don't remember what it was originally, but not Rioux or Rivoult.
Brittany is devided into at least three departments (Cote d'Armor, Finistre and Morbihan), and he could have come from any three of those areas. Without an exact place of birth and his mother's maiden name, your efforts will be a shot in the dark. Plus, records will be in French and unless you know the language, and the intricacies of the various French Archives and searching in the French records system, such a search will take a great deal of time. It is not impossible however.
I would suggest you contact André Lafargue on this message board (see the message entitled "CUSINAGE" which should be spelled Cousinage) who may have connections to friends in France who could possibly help. While I know a lot, I am no expert. At least you have Pierre's year of birth and that might be a start.
Also, have you posted at the BRITTANY message board? Perhaps someone there can also help.
Good luck, Dennis
Here's a link to the various French Archives; Click on the various sections till you find the 3 departments listed above:http://geneinfos.typepad.fr/geneinfos/nouvelle_carte.html