Does child support have to be filed with our income taxes? Is it considered taxable income?

It's kind of a multi-fold problem because the order was written wrong, his ex received over $1,200 in overpayment which she was ordered to repay; they are also still withholding from his unemployment check because the order wasn't written properly. Between those two things, they're showing he received almost $2,000 more last year than the court ordered amount - it wasn't support, it was just him getting repaid his own money from her and unemployment.

Child support is neither taxabe income for the recipient nor tax deductible for the payor. So all of the over payments/underpayements are NON-events tax-wise. It has ZERO effect on his tax return.