Apply for TD status for your child now at POE. Bring original long form birth certificate, showing names of both parents, assuming your child was born in Canada, to show he is your son.
They will issue TD status until expiry of your TN or expiry of your child passport, which ever comes sooner. If your son's passport expires Nov 2021, which is sooner than your TN expiry, the TD status will be valid till Nov 2021. Six months prior to Nov 2021, ie, May 2021, apply for a new Canadian passport for your son, apply by mail from US, ensure the Photos meet the Canadian passport requirements (50 x 70mm, rather than US passport requirements of 2" x 2"). Once your son has a new passport, apply again at POE and they will issue new TD status until the expiry of your TN status.
If you try to obtain new passport for your son now, they will likely not renew as he still has over one year before expiry. You can try in person and see if they will issue a new passport to your son. You need a valid reason to renew before one year of expiry. They may not consider this a valid reason.