for application purposes, he is not a "dependant" in the sense that you need to fill out applicant forms in his name as he is not seeking PR status in canada. As the primary applicant, you do have to fill out IMM5406 (Additional family form) for yourself, and list your son as your child. You do not need to fill out a separate form for him. You need to submit proof of his canadian citizenship in the application, so IRCC knows he is not eligible for PR.