Yeah, you are in a bit of a tight spot. Perhaps, and I don't know if this will work or not because I don't know that much about this kind of a situation, there is a way for you to file as common law, because if you have in fact lived together for a year or more, you are common law for CIC's purposes', but you will NEVER be married as far as CIC is concerned because your spouse is still married to his first wife. Maybe you could file as common law and also give them a secondary declaration with a well thought out, written explanation that in your home countries customs you are legally married, but you do understand these customs are not followed, nor are they acceptable in Canadian society, but you would like to include both him and your child in your application for immigration. I would make sure they have a well developed timeline for how this all came to be and that it is not something that happened in the last 6 months or something so they don't view it as a convenience issue.
As I said, I'm not sure if that will help you or not, but it certainly can not hurt. If you don't disclose him or the child then it will cause you more harm than good later on down the road.
Best of luck to you.