There are two tax benefits for claiming a disabled dependent. You can deduct his medical expenses, and if your son is mentally or physically incapable of self-care, you can claim child and dependent care credit if you paid any expenses to an individual so that you can work or go to school full time.
The tax benefit for a disabled dependent is also in the fact that you can claim him as your dependent regardless of his age. Other than above, there are no specific deductions just for being disabled.
As to his SSI benefits - they are never reported on your tax return. And if that is your son's only income, he doesn't have to file his tax return, either.
