If his parents (or anyone else) do NOT QUALIFY to claim him as a dependent for 2021, and he meets the other requirements, he can claim the $1,400 Recovery Rebate Credit #3 on his 2021 return.
If his parents DO QUALIFY to claim him but DO NOT CLAIM him, he DOES NOT QUALIFY to receive the $1,400 RRC.
Was he a student for any part of any five months in 2021?