Why did you file Married filing Separate? Child support does not affect your tax return. Filing Joint should be better. Does he owe back Child Support and they keep his refund? If you file Joint you can file a Injured spouse form to protect your part of the refund. You can amend to change it to Joint.
Many people think they come out better when filing Married Filing Separate but they are probably doing it wrong. If one person itemizes deductions on Schedule A then the other one must itemize too, even if it's less than the standard deduction, even if it is ZERO! And if you are in a Community Property state it can be complicated to figure out.
And there are several credits you can't take when filing separately, like the
EITC Earned Income Tax Credit
Child Care Credit
Educational Deductions and Credits
And contributions to IRA and ROTH IRA are limited when you file MFS.
