Rules for claiming a dependent and eligibility for SSI are different.
See IRS publication 501 page 11 - http://www.irs.gov/pub/irs-pdf/p501.pdf
Tests To Be a Qualifying Child
-- The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
-- The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student, or (c) any age if permanently and totally disabled.
-- The child must have lived with you for more than half of the year.
-- The child must not have provided more than half of his or her own support for the year.
-- If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
That means you provided more than half of your child's support in form of housing, food, etc
Because your son receives SSI - assistance based on his needs - http://www.ssa.gov/ssi/USSI2008clean.doc
On his application he or a guardian should report all assistance he receives - including your support in in form of housing, food, etc
To determine if your assistance would affect his eligibility - use this link - it will take you to the Benefit Eligibility Screening Tool. By taking 5 to 10 minutes to answer a few questions, you can find out if his eligibility for SSI or other benefits would be affected.
You may also consult with your case worker.