How old is he? If he was 17 or older in 2020 you don't get the 500 or 600 either for claiming him as a dependent.
Then for his return. Sorry the Recovery rebate credit is really really based on his 2020 return and situation. Since he is now a dependent he doesn't qualify for any of it. But he does not have to pay the $600 he got back. They sent out the first 2 payments using his 2018 or 2019 return just to send it out fast. It was an Advance to be reconciled on your 2020 return. Lots of people got the payments that don't qualify now based on 2020 and they don't have to pay it back.
