"So if the parents did not get the additional stimulus payments because the child was 23 and a college student"

Stop using any qualification from any year prior to 2020 in your analysis. They Don't Matter Now.

"nor did he then he can claim on this years taxes that he did not receive them since he is not a dependent now and receive the 1800 as a refund?"

You are making this too complicated.

Fill in the 2020 tax forms properly, and if a person got any Advanced Payments you need to show how much they already got, because they got money already. If they also qualify for more, they have a Refundable Credit coming to them.