That's not really the issue. When filling out the application, you swear that you have listed ALL of your family members (including your children) and have done medicals for ALL of your children (whether they intend to come to Canada or not) and that any child NOT listed, will never be able to come to Canada. So, by not listing the child (because he didn't know about it), he cannot bring the child to Canada.
The burden of proof would be on the poster to prove that he had no idea that he HAD this child to begin with and was not lying (misrepresenting) himself when he applied. He may be able to do this on humanitarian and compassionate grounds, as PMM said, however, these cases are rarely successful. A good lawyer may be able to help the OP prove that he did not know about the child (gather proof such as sworn letter from the mother stating she did not inform the OP he had a son, etc) and may be able to help him with his case, but as a general rule, the child is not allowed to come. In cases like this, I would hope that an H&C appeal to CIC would be successful, but you just never know.
I wish the OP good luck and hope that you get your son. You will almost certainly have to prove paternity before an H&C will be considered, so I would work on getting that in order. CIC only accepts certain labs that do this testing. I suggest you and your lawyer work on that... they will want to know the child is actually yours (and I'm sure you do too!)