If your return keeps being rejected when you claim him and you believe you can claim him and no one else has claimed him you will have to mail your return. 

If you took him off and got your return Accepted then when the IRS is done processing it you can file an Amended return.

How to amend a 2020 return

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439

If you filed your tax return without a dependent, then you have to prepare and file an amended tax return Form 1040-X.

You will check the box in Part I that you are changing dependent info, but see the tricky instruction right there on that checkbox.

If you e-Filed 1040, as you say, you get one e-File of 1040-X.