suggest running through this IRS tool so there is no ambiguity
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
further, he is only eligible for the stimulus if you CAN'T claim him as a dependent. He should file a 2020 tax return and $1200 should appear on Line 30 of Form 1040.
If you CAN claim him, even if you choose not to, he is NOT eligible for the stimulus. And if there was no tax withholding on the internship (and he received a W-2), there may be no need to file.
