First of all, if the child's father is actually receiving Supplemental Security Income (SSI) benefits, there are no additional benefits for dependents. It is possible that he is receiving SSDI, which is Social Security disability benefits. (These two programs are commonly confused, even by attorneys.) If he is receiving SSDI and worked enough to have earned dependent benefits, then he should apply for those benefits, and they will be paid to the adult who has custody of the child. If your son's father refuses to do that, you may need to petition the court that ordered the child support to order him to make that application. He should be anxious to comply as those benefits can satisfy his child support obligation. The fact that nothing has been done, suggests that he is either receiving SSI, or did not work enough to have dependent benefits.
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This answer is offered as a public service for general information only and may not be relied upon as legal advice.