has your husband's application been approved and has he landed for PR already? if so, i'm afraid he won't be able to sponsor his son at all, since he legally gave up his right to in order to be approved.
if your husband's application is still being processed, then you want to send updated forms to his visa office with this new information as well as a letter of explanation ASAP so he can be added to the application as an accompanying dependant. you can do this by following the directions received in the sponsor approval letter or by going to the IRCC website and submitting the forms and explanation letter by case specific enquiry.
if you have not received sponsor approval yet, then wait till you receive that letter and submit the updated information.
your husband should not submit a passport for visa processing or land as a pr until he receives confirmation his child was successfully added to his application. not confirming this can easily bar him from sponsoring his child ever.