Thanks so much for this detailed responseRead the instructions carefully. For the additional family information form IMM5406, only dependent children over 18 submit a completed form (info on the child regardless of citizenship is entered on that form for the PA/mother).
On IMM0008 (generic application form), you include all dependant children who are not already permanent residents or citizens; you should not include your son here because he is a Canadian citizen. This is true even if he has not yet received his citizenship documentation - he is a citizen even if he does not have documentation yet (note that I am assuming that you have correctly checked that your citizenship is such that he will receive, i.e. you are not second generation born abroad, etc).
Please also refer to the Checklist form (IMM5533) Section 3 - it tells you to apply for Canadian citizenship for the child, and asks you to include a copy of the documents for your son - including one of passport, citizenship certificate, or proof of application for proof of citizenship (i.e. your citizenship application for your child). You can include a short explanatory note here, i.e. you have a son together and have applied for ctizenship, copy of application attached.
So to sum up: no additional form for child; info on child is included on mother's additional background form IMM5406; info on child is not included in generic application form (i.e. the child is not applying for PR as already a citizen); and you should include something showing you expect your son to receive Canadian citizenship or already has it (you do not need to wait for him to receive his Canadian citizenship).