Greg, I'm going to save you some time, I've copied and pasted your post here.
From your post apparently your CS order has been in existance for a long time. It is way too late to contest paternity or to get the order stopped. The current support will stop when the child becomes and adult, the arrears will stop when they are paid off.
"TX..i am paying arrears child support to a kid that is not mine..how do i get this stopped? this kid is 17 yrs old and i have been paying on him since he was small..i have had enough and need to know the legal way to get it stop asap..will provide more info as needed...thanks"