If reading correctly your ex-husband signed as well? ATIP will ask for "A recent Canadian court document showing custody agreement" (this is a standard message in applicable cases) if both parents are unable to provide consent. If both parents have signed the consent form then you are all set.
ATIP staff verifies signatures provided on the consent form with what they have available at their end. If there is a mismatch ATIP will contact you for an ID. You can proactively provide it and it will not hurt the processing in anyway. During COVID-19 if hand signatures are not possible ATIP will accept electronic signatures as well but only if accompanied with a govt. issued ID or an audit log (companies like DocuSign etc provide an audit log)
Thank you for your response. Yes, he did sign it but as per my knowledge ATIP doesn't have any record of his signatures except in IMM5604 form which was an ADR (which also includes his ID)