You say he has separate housing. That means he does not live with you. That means he cannot be your qualifying child (QC).
That means we go to the qualifying relative (QR) rules, to see if he can be your dependent. The support rule for a QR is different than for a QC. For a QC it is that he did not provide more than 1/2 his own support. For a QR, it is that the taxpayer (you) provide more than half his support. So, his SSI income works against you, because it is third party (welfare) support and not support provide by you.
Full rules at
https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
