I agree with RobsLuv but this is how I would play it:
1. Proceed with the PR application so you can get your kid's PR, since your husband is the Principal Applicant, if you withdraw his you have to make your child the primary applicant and it'll pretty much restart the whole process. Plus, since both of you have custody of the child, you are going to need his permission to see your kid.
2. Prepare a legal separation and a custody battle starting the moment your child gets his PR card, think that even if he does go on welfare and binds you to an 18000 CAD debt, the money you'd pay for an international custody battle would be much greater, if only on plane tickets and lawyers.
OR
Try to get your kid into Canada, do not let him leave and withdraw your soon-to-be ex-husband's application. Then resume his PR application now with him as the Principal Applicant