As soon as your new wife has kids of her own then she'll start treating your son bad and if he does anything even a tiniest thing to her kid then your older son from previous marriage will get it. Why do you think there is bad things said about step moms? there can't be a fire without a spark. Don't be in haste to re-marry, please think of your son's future not just your selfish needs. I have seen first hand, no matter how good a step mom is she's still a step mom and there will be discrimination. Unless you marry someone who can't have kids of her own but where will u find that kind of girl?
And even though you are not together with your wife anymore, she is still your son's birth mother, noone will love him more then his own mother and a child needs both mother and father growing up. So I think you should let her come to Canada first and let her change her status so your son can have love of his own mother growing up, then you can re-marry after that if you choose to.

Too bad the relationship between you and your wife did not work, but don't make the poor innocent kid pay the price for it.