Does SSI count a back child support payment as income?

I know that SSI counts regular child support payments as income but the site says nothing about back child support. My son receives SSI and just received a back payment of child support. It was the first payment of child support his father has made in 8yrs. I am uming it was his income tax return or that of his new wife if they filed jointly since I know that my son's father only works under the table so that they don't garnish his wages. I have been calling Social Security to find out but I am on hold forever or I get a busy signal at our local office and I would really like to know.