"He turned 18 on 12/24/2020 and began a high paying automotive job."

Is he is not able to be claimed as someone else's dependent for 2020? As the link I gave shows, this means No Longer Qualified to be claimed. That would be different than "not being claimed."

And the amount per Adult who files their own tax form is $1800.

That's why you need to read those links, work the dependent wizard, and go through the process for the analysis for your own understanding. Bookmark those links.