Child graduated June 2020, now self supporting/working & I'm providing no support since graduation. Will not be claiming him as a dependent. Can he be a former dependent?

There is no such thing as a "former dependent" on a tax return.  You either claim the dependent or you do not claim them.   If he can be claimed then on his own return he has to say he can be claimed as a dependent.   If he does NOT meet the criteria for you to claim him, then you do not enter anything at all about him on your own tax return.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent  

You might still be able to claim him for 2020 if he was under 24 no matter how much he made since he was a full time student.  But then he can't claim himself.  And even if you don't claim him he still has to check the box that he CAN be claimed.