@Hlovering,
If this were me, I would call the IRS, explain that I am the custodial parent and that I am not receiving any child support. I would say clearly "my ex-wife filed this in error" and that I intend to claim the child on my taxes.
Then I would ask them what I can do to continue with the child on my tax return. I just googled, the non-custodial parent is in the wrong in this case.
He might be able to handle this issue with the IRS without the need for a lawyer. I would be polite, but insistent... and not commit to anything until they said I was correct.
That would be me. It is probably still a good idea to talk to a lawyer (but if all you care about are the taxes... you might not need one).