let's back up a little bit.
Did he earn more or less than $4300 in 2020?
the first question is whether your son CAN be claimed by you. Whether you do or not is not material. if he CAN be claimed by you, then that is the end of the story as he must check that box on HIS tax return and he is not eligible for the stimulus. And that is true, whether or not you claim him. You always have the option to claim him, but from his perspective, whether you CAN claim him is the same answer whether you do or not.
Suggest running through this tool to confirm whether he is your dependent or not
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
if more than $4300 you can NOT claim him, In that instance he is NOT to check the 'CAN someone claim' you check box and he'll see $1800 on Line 30 of Form 1040 for the stimulus payments.
if less than $4300 (and presumably you provided more than 50% of his financial support), he has to check the box stating someone CAN claim him and he is not eligible for the stimulus.
