Q: Did C pay for more than half of their living expenses and other support? My son is 20 & on SSI. He pays his fair share. "more than" meaning what?

As rjs states, it depends on whether the child provided more than half of his own support.  Assuming your son meets the other tests to be a qualifying child (see below), the disability social security income that is actually used for his support is considered as provided by the child.  

According to IRS Publication 501: 

"Social security benefits. 

...If a child receives social security benefits and uses them toward their own support, the benefits are considered as provided by the child."  

"Total support includes amounts spent to provide food, lodging, clothing, education, medical and dental care, recreation, transportation, and similar necessities."

So if you divide all of his disability payments that were actually used toward items listed as support by the total spent on items listed for his support and that is more than 1/2 (.5) then you answer "Yes" he provided more than 1/2.  If not, then answer "No". 

If he is not your qualifying child, then he could still be your dependent as a qualifying relative (see below).  In that case you would need to determine if you provided more than 1/2 of his total support. 

Qualifying Child tests:

• The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, or stepsister, or a descendant of any of them.
• The child must be (a) under age 19 at the end of the year and younger than you (or your spouse if filing jointly); (b) under age 24 at the end of the year, a student, and younger than you (or your spouse if filing jointly); 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 the child’s own support for the year.
• The child must not be filing a joint return for the year (unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid).

Permanently and totally disabled.

Your child is permanently and totally disabled if both of the following apply.

• Your child can't engage in any substantial gainful activity because of a physical or mental condition.
• A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.

Qualifying Relative tests:

• The person can't be your qualifying child or the qualifying child of any other taxpayer.
• The person either (a) must be related to you in one of the ways listed under Relatives who don't have to live with you, or (b) must live with you all year as a member of your household (and your relationship must not violate local law).
• The person's gross income for the year must be less than $4,700. (non-taxable SSI is not included here)
• You must provide more than half of the person's total support for the year.