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There are two sets of rules for dependents.

Your son can be your qualifying child dependent if:

1. He is permanently and totally disabled. For tax purposes, disabled means unable to perform gainful work (work for money).  Many blind people can perform work, but if the social security administration has determined he can't work, that will probably satisfy the IRS.
2. He lives in your home more than half the year.
3. He does not provide more than half his own support.  Here, his gross income is not part of the test, just whether or not he provides more than half his own support.  It is not required that you prove you provided more than half, just that he did not provide more than half.  SSDI counts as support he provides himself, assuming he spends it on his financial needs.  (If he puts it all in savings, or spends it as child support for someone else, then it is not support he provides to himself.  But sending on groceries, entertainment, clothing etc. is support he provides to himself.)

Or, your son can be your qualifying relative dependent if:

1. You provided more than half his support.  Here, you must prove you provided more than half, not just that he provided less than half his own support.
2. His gross taxable income is less than $4700.

For this type of dependent, it is not necessary to prove disability, and not even necessary that he live with you, as long as you provided more than half his support.