If you can claim him, you are not required to do so, whether he qualifies under the qualifying child rules or the qualifying relative rules.
But, on his return, he must still check the box, on form 1040, that says he CAN be claimed as a dependent. That means he is not eligible for tax benefits that non dependents get. For 2020, in particular, that means he does not get the $1800 (1200 + 600) 1st and 2nd stimulus.
