You are asking a series of very complex questions that I will attempt to address very quickly. If you want to know the answers to these questions in greater depth then you need to meet with an attorney in person and pay for their legal advice.
If you are married then you cannot get child support from your husband. If you live together, he is supporting your children. The TX A G office would not accept your case.
Your husband can reduce his child support for his one child by the 2 that he has with you. The next time his ex files to modify his child support he needs to mention that he has 2 additional children. His percentage will automatically be reduced.
If you ever divorce him, his eldest child will always come first when it comes to child support. You knew that this child existed when you married him and you decided to have additional children with him. I'm sorry for this very harsh statement, but his obligation to his eldest child is #1... before you and your children.
And, you don't have to buy this child anything. If his mother is not providing for his care, then your husband needs to consider filing for custody of this child. He needs to talk to a tough, smart family law attorney about gaining custody of his eldest son. You live in the Houston area and there are plenty of father's that gain custody of their children in the Harris County family courts. If the mother is negligent, the burden is on the father to prove it but it can be done. Talk to an experienced family law attorney about the odds of gaining custody.
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