You can certainly try to seek a court ordered DNA test or perform a home version, but that is probably not going to affect a previous final determination of paternity unless the ex steps up to claim he is the father. You treat him as your son. Continue to do so.
So, you've never had a paternity test but are ordered to pay support. That means you either 1) signed the Voluntary Acknowledgment of Parentage (admitting you're the father) when the child was born or 2) admitted in court that you're the father.
Assuming either #1 happened more than 60 days ago or #2 happened more than 30 days ago, you're stuck. So, CONGRATULATIONS! You're the father! Even if you do a DNA test (you can pick one up at your local drug store) and find that you're not the father, biologically, you'll still be the father legally -- and in court, that's all that matters.
If #1 or #2 happened within the time frames mentioned, above, there's still a way out. Otherwise, congratulations!
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