IF you are not claiming the child then delete them in the MY INFO section.
As for the 2019 return ... you will need to amend it if the original return has fully processed.
How did you 'accidently' claim your son last year?
If you CAN claim him, you don't have to.
but from your son's perspective, if someone CAN claim him, even if they don't, he has to answer that way. (and if someone CAN claim him, even if they don't, he is not eligible for the stimulus.... if that is why you are really asking)