Sounds like you need to amend your 2019 return and include him for the WHOLE YEAR. Even if the baby was born on Dec 31 you get to say he lived with you the WHOLE YEAR.
https://ttlc.intuit.com/community/amend-tax-return/help/how-do-i-amend-my-2019-turbotax-online-retur...
Did you file 2020? Did you get the Child Tax Credit for him? If your 2020 return gets accepted you might need to amend 2020 also.
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439
