Will I be required to pay child support for a child that is not mine?
Under Texas Family Code (FC) section 160.204 you are presumed to be the father of the child since apparently he was born during the marriage. The only way to rebut that presumption is either to (1) litigate the issue of paternity or (2) for you to file a denial of paternity AND for the real father to file an acknowledgement of paternity at the same time.
You have a problem here because generally a presumed father must file the litigation to challenge paternity no later than the child's fourth birthday. FC § 160.607(a). The only exception to that is in FC § 160.607(b):
"(b) A proceeding seeking to adjudicate the parentage of a child having a presumed father may be maintained at any time if the court determines that:
(1) the presumed father and the mother of the child did not live together or engage in sexual intercourse with each other during the probable time of conception; or
(2) the presumed father was precluded from commencing a proceeding to adjudicate the parentage of the child before the expiration of the time prescribed by Subsection (a) because of the mistaken belief that he was the child's biological father based on misrepresentations that led him to that conclusion."
In short, you can challenge paternity now, but first you must convince the court that you were under the mistaken belief the kid was yours because your wife made misrepresentations to you that lead you to that conclusion. Then you'd be able proceed with rebutting the presumption and proving the kid is not yours. If you succeed in rebutting the presumption of paternity and getting the court to declare that you are not the legal father, then you'd be off the hook for child support.
But you'd likely also be precluded from seeking any visitation or custody of the child, and the mother could then deny you a relationship with the child. Is that truly what you'd want? Think about the consequences and the effect on the child of suddenly losing contact with the only father he's ever known.
If you wish to proceed with the challenge to paternity (whether or not you decide to divorce) then you ought to see a family law attorney in Texas ASAP. It won't be easy and won't be cheap if your wife fights you on this. Now that you know you are not the father, sitting on that challenge for some period of time may work against you in challenging paternity later. So if you're going to do it, you should do it soon.