Child support arrears accrued after children grow up are still owed, yes. It might be possible to garnish disability payments for child support, but the payments will necessarily be rather low. He can't be garnished for more than 25% of his monthly income, or be left with less than $936 a month (that's for working full-time; different limits may apply for SSD. You should consult with an attorney with experience in support collections.).
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