No, it's probably not possible. If one of your dependents fails the medical exam by having a condition that would be too expensive for Canadian healthcare (more than approx $6500 per year), then the entire app is rejected for everyone. There are no exceptions to this.
If you went ahead and applied anyways, when it comes time to assess the medical exam you would likely get a procedural fairness letter explaining they are preparing to reject your app due to excessive demand, and giving you a last opportunity to try and counteract their claims. In most successful cases the applicants show the costs to treat the medical condition will not be as much as IRCC thinks, or they have some legitimate plan to offset the costs.
See here:
https://www.canada.ca/en/immigratio...ted-procedural-fairness-excessive-demand.html
You mention you have insurance already, but the problem here is that once a PR there is absolutely nothing stopping you from cancelling the insurance and putting your child into the Canadian healthcare system where all his costs will be covered by the taxpayers. It's because of this the app would most likely be rejected. Perhaps you can research if there have been any successful cases involving dependents with down syndrome, but due to the incredibly expensive health and social services cost this condition entails, I highly doubt it.