If your written agreement specifically says that the support payment is for your son, then you do not include the amounts in income and the child's father does not deduct the payments.
As per the CRA's guidelines on support payments, "Under court orders and written agreements made after April 1997, any support amount that is not stated in the order or agreement as being only for the support of the recipient is considered to be child support. These amounts are not deductible by the payer and do not have to be included in the recipient's income."
Full details can be found on the CRA website at https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/p102/support-payments.html.