Don't panic send them an explanation letter saying that you didn't disclose your son maybe because you didn't know that he was born or his mum didn't want him to join you in Canada. In most of the case the ex spouse refuses to let their children immigrate with their father to Canada and therefore refused for their children to be examined.But it seems like in your case you didn't even declared your son. It seems like you made a conscious decision to exclude (either by not declaring and/or not having your son examined), then in this explain to the Officer the really reason why you didn't disclose. I hope you understand.