I can't speak for Canada as I am in the US. I can tell you that is common for a designer to be able to say he designed a logo. For example Rob Janoff designed the Apple logo and Paul Rand designed the IBM logo.
Since you want something "out of the ordinary", that is the author to not be given credit for his work, it would have been best to discuss this up front before the work began. Further since it is standard practice for any creator (author, painter, graphic artist) to "sign" his work, it is natural that your graphic artist would assume that would apply in his case as well. Since he asked you about including it in his portfolio, it is clear that he did not initially intend to do anything behind your back.
If you push things, the issue will be what rights exactly did you purchase in the work? This is why movie contracts go to many pages. Initially an artist owns his work, and generally the rights in the work which are transferred are limited. So for example he might contend that what was transferred was the right to use the image as a corporate logo only. This is like hiring a portrait painter to paint your likeness. By default what you get is the painting; you do not get the rights to sell prints of the painting or use it on TV for example.
I am curious as to your objection to having it be know that he is the creator of your company's logo? Since Apple, IBM and virtually all major companies have no problem with the world knowing who created their logos, it wouldn't be an issue to me.