is it true , when the costody parent leaves the u.s. dose the child support stop

No.  Child support will continue until the child reaches 18 or graduates from high school whichever is later, but not beyond 19 1/2 years of age so long as the child is residing with the person receiving the support.  This response is general in nature and is not legal advice.  No attorney client relationship is formed by it.  Good luck.