If he was receiving Social Security Disability benefits and you claimed him as a dependent that would have no impact on the benefits he receives.
However, since he receives SSI you should speak with a SSI representative to see if claiming him as a dependent would affect his SSI.
Also, go to this SS website for information on WHAT IS IN-KIND SUPPORT AND MAINTENANCE? - https://www.ssa.gov/ssi/text-living-ussi.htm
In-kind support and maintenance is food, shelter, or both that somebody else provides for you. We count in-kind support and maintenance as income when we figure the amount of your SSI benefits. For example, if someone helps pay for your rent, mortgage, food, or utilities, we reduce the amount of your SSI benefits. Receiving in-kind support and maintenance can reduce your monthly SSI benefits as much as $300.33, depending on the value of the help you receive.
