First of all.......sorry about your dad. Sounds like he struggles to keep his head straight.
Second, I don't think I've ever heard of anything like this before. I would call the bank and ask them about it to make sure it's legit and the people who sent the letter. The Government typically waits until you do your taxes before they take your money because that is "guaranteed money", so just when you think your getting a $5000 check, you get a $100 check because the government takes their shit.
Third, the government (I believe) will contact his employer and set up payments if he really owes back child support. Again, "guaranteed money" because he worked for it.
When I got married my chick was on assistance for 5 months. I had to pay back everything through my employer.
Hope that helps.
5000