"And now back to my question"

And now we fall back to "not enough info." Your client's original concept was incorrect and you would explain why.

"My client made too much in 2018 and 2019 and did not receive a stimulus check in Apr 2020."

Your client still could make less money in 2020 (stop working now?), because everyone has yet to file 2020 to reconcile the Stimulus as an Advance against Actual. This parent would not get any money for the son that is this old, of course, no matter how he intends to play the game.

There's still time for your client to attempt to manipulate the system.