Forget about the first 2 for 2020.
Ok so he did get it. He can keep it. Then he should say Yes he got the 3rd Stimulus 1400. It was right for him to get the 1400 check. Why did you think it was an error? It will ask him if he got it. If he says No and tries to claim it again the IRS will adjust his return because they already sent it to him. If he can't cash it now I guess he can get a replacement. See the first question here. Say it was lost.
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-round-economic-impact-payment-top...
Then on your return yes you say you got 4200 which is more that the 2800 you qualify for. So you don't qualify for any more on your tax return. And you don't have to pay his back.
The 3rd Stimulus payment is really based on 2021. The IRS just sent the checks out using 2020 or 2019 to get them out fast.
See IRS Topic B first question
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-b-e...
