I have a quick question:
My Sister lives in CA and her Ex in Nevada,
Their Son is to turn 18 in 2 years yet her ex hasn't paid child support from the years 2007-2010
She has let him know of the court order he stated he never received any documents... Now that she sent in the proof of the court order he is stating he gave money without recording it anywhere through out the years to the child ( which is true) so he doesn't owe anything.
What can he do if he doesn't have proof what can my sister do for him to pay for the skipped years.
He never came to court and claims he never received any paperwork that he signed.

He was aware of the court case, but simply never got the support order? He should have wondered what was happening in the case and looked into it. It's probably too late for him to do anything about it now. And, especially if he signed the paperwork, he will probably be laughed out of court.

What state did the order come from? He may owe interest on arrears. Also giving money to the child is a gift. Giving money to the mother may or may not be considered child support.

He never showed up to court and claims he never signed anything, but since he was giving money a hundred here 200 there for birthdays christmas he claims he never knew of the court order and he never received anything to go to court. Which is not true according to my sisters attorney yet it is true that he never signed for anything because he was never around. He is threatening to get an attorney and fight my sister to her bones because of this. The order came from Orange County, in California.

Since the order is out of California then any arrears are charged with an additional 10% interest. That is also a compounded amount. Depending on the original order he could be in the hole for tens of thousands of dollars in no time.