Thank you for your question. Please permit me to assist you with your concerns.
The statute that governs your obligation is Sec. 154.001 of the Texas Family code. Under Texas you remain responsible for the child he turns 18 OR graduates, whichever occurs later. In your situation if the child was held back a year, he is not considered to be 'emancipated' or independent which is likely why you are being compelled to provide more support. However if he fails to graduate again, what you can do is file a petition, on your own, for 'emancipation'. In this petition you can claim that the child is now old enough to care for his needs, and that since he has chosen to not graduate, he has voluntarily emancipated himself from further assistance. While an attorney can help, you should be able to file this on your own and you should be able to convince the courts to drop this obligation against you.
Here is the relevant portion of the statute:
Sec. 154.001. SUPPORT OF CHILD. (a) The court may order either or both parents to support a child in the manner specified by the order:
(1) until the child is 18 years of age or until graduation from high school, whichever occurs later;
(2) until the child is emancipated through marriage, through removal of the disabilities of minority by court order, or by other operation of law;
(3) until the death of the child; or
(4) if the child is disabled as defined in this chapter, for an indefinite period.