Canadian border crossing and American criminal record

I work in the U.S. and would like to bring my American boyfriend home to Canada for a visit. The problem is that he has a criminal record, and it hasn't been 5 years since the end of his probation. According to the "criminal rehabilitation" documents for Canada, he's not eligible to apply until 5 years after the end of his probation.

A friend of mine who used to work customs for the Federal government, was telling me that unless they decide to check for a record, they wouldn't see it when we cross the border. I'm worried that if we tried crossing, and he gets denied, he would have a harder time to get the criminal rehabilitation later on. And maybe even make my re-entry more complicated in the future.

Does anyone know what are the chances he gets denied access?
As anyone been in the same situation?