Dependent Adult

Because he is disabled, you would not have to consider his disability income to qualify to claim him as a dependent- he would be your qualifying child.  Foe Head of Household purposes, you would only be required to show you paid for more than half of the total household expenses.  

Under proposed Treasury regulations, if you received Temporary Assistance to Needy Families (TANF) payments or other similar payments and used the payment to support another person, those payments are considered support you provided for that person, rather than support provided by the government or other third party.  Although this has not been finalized, once it is you will be able to use his Social Security Disability as provided by you.  

There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test.

A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:

1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled (regardless of age)
2. He did not provide more than 1/2 his own support. Scholarships are excluded from the support calculation
3. He lived with the parent (including temporary absences such as away at school) for more than half the year

So, it doesn't matter how much he earned. What matters is how much he spent on support. Money he put into savings does not count as support he spent on himself.

The support value of the home, provided by the parent, is the fair market rental value of the home plus utilities & other expenses divided by the number of occupants.

The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf