out of his disability check. My son's father was way behind due to being disabled and unable to work, and when he was finally approved for disability the first thing social security did was take the several thousand he was behind out of the back disability he received in a lump sum. Had my son not been of age at that point, they would have continued to take it out of his disability each month. I actually felt bad for him, 'cause, unlike so many other deadbeats I know of, he wasn't deliberately trying to avoid paying, he was genuinely physically unable to work and he suffered horribly financially before he was finally approved.
It's my understanding that they automatically check for things like back child support and taxes before approving and disbursing disability, so not sure why that wouldn't have worked in your case.