Child support enforcement is individual to each state. That said, there is the Uniform Reciprocal Enforcement of Support Act. You need to find the state agency in your state that handles child support enforcement.
In talking with my local child support office they have informed me that SSI (not social security) is considered public assistance and receiving it puts you in the poverty level. In TN and IN they don't place an order for support if there is a disability determination and SSI is the only source of income. If he were getting "regular" social security disability - it would be a totally different story. Contact your local child support office that holds your order and ask them.
All you have to do is go to the social security office and talk to them. They will be the ones to pay you. They will even pay back support from when the child was born. Trust me I did it they send me 1020.00 a month. It wont come out of his money it will come from them. JUST GO TO YOUR S.S OFFICE. YOU wont regret it!!!!!!!!!!!!!!