Your fiance won't be able to go to Canada until he is considered rehabilitated. Depending on the offense and his actions since that time, it could take a few years or it might never happen.
Here is a link to CIC's website concerning rehabilitation..
http://www.cic.gc.ca/english/information/applications/rehabil.aspYour fiance has the option of filling out the form and taking it to the Canadian consulate/embassy to get an opinion on what would be required to be considered rehabilitated. I think it's a free service.
Your fiance should do this before you start thinking about immigrating, of course.