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
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.