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.  

https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-a-2019-return-in-turbotax/00/123845...

are you sure you do not qualify to claim your son?  if you do you don't have to claim him. but if you can even if you don't he must mark his return that he can be claimed. 

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)