There is a difference between support and income. Food stamps are not income. But they do have a monetary value and count as support being provided by a third party. If you get more than half of your support from third party sources, then your son can't be providing more than half himself. Support is things like (not all inclusive)
Food
Clothing
Housing
Utilities
Transportation
Entertainment (within reason of course)
Medical expenses (to include insurance. If getting subsidized healthcare, the subsidy value is 3rd party support.)
Education costs.
