Your husband will need to continue to pay the child support until his son graduates (or leaves) high school per the Court Order. After your child is born, you may seek to lower the child support due to a change in circumstances (duty to support another child).
I agree with the other attorney. The teen-ager having a baby does not make him an "adult" in the eyes of the laws of Texas. He is a "child" having a "baby". Your husband must still pay child support until he graduates from high school or turns 18 - whichever occurs later. Your husband might want to consider lowering his child support after the new baby is born but it might not be worth it - you would need to talk to an attorney & let the attorney calculate it. If your husband's income has gone up then it might not be worthwhile to pursue this matter.