You cannot add a child born in 2021 to a 2020 tax return if that is what you are asking to do.   You will be able to enter his information as a dependent on your 2021 tax return next year.

But....

Congratulations on your child born in 2021!   There will be a portal on the IRS site where in August or September you can enter information for a child who was born in 2021 to get the monthly child tax credit advance payments.   You will need your child’s Social Security number.  The portal is not ready just yet.

https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...

Congratulations! Yes, you can claim your child on your 2021 tax return. 

Your child must live with you for the year, you must provide at least 50% of the child's support and you will need to apply for a social security number. 

Ok thank you