If he's paying child support, and it's not to your or your kids together, then there are only a few reasons for that:
- He's pocketing the cash.
- It's a lie that he paid anything to anyone (almost identical to #1 but without writing a bogus check first).
- He's sending money somewhere else.
And if it's #3, maybe there are other children in the picture.
Talk to your lawyer - I assume, hope you have one? - as it's probably going to be that you have no claim to child support, seeing as parental rights were extinguished. Then the "child support" is more like a gift, and you'd have no right to demand such.
But TALK TO YOUR LAWYER, as I may be wrong and certainly it's your lawyer's job to research such things.