What do I do to stop child support payment now that my son has turned 18 and has finshed high school

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

First what does your support agreement say? Does it say it stops at 18 or when he finishes HS?

Are payments goming out of your paycheck automatically? Or do you pay to an agency or directly?

What state is this in ? Some states like NY and Ind run until 21 depending on when the order was written.