Since he does not live with you, you may be able to claim him as a dependent if he is your qualifying relative. To meet this test, you must provide more than half of his support and his taxable gross income must be less than $3,650. The SSDI and SSI are not taxable so they don't count for this rule. You will need to add up the total cost of his support to determine if you pay the larger share.
Thank you for your question,
Yes, if you pay more than the total he receives in income, then you can claim him.
Since food is counted in the support calculation, then yes, the amount he received in food stamps is added to the amount he paid for his own support (his ssi and ssdi).
Hope this makes sense,