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It is true that your son will receive support as well as part of his father's SSDI. But if he has received a fully favorable decision, he is probably owed past due benefits... in which case the child support can be garnished in a lump sum. Even if it is not, child support can be garnished from SSDI.
As far as making it more difficult... I don't see how that would be the case. First of all, child support has no impact on his SSDI determinations, second, he already has the fully favorable decision. Having dealt with thousands of SSDI determinations, I really don't see the difficulty here. I see no reason why forgiving his child support would have any effect on anything.