procedures for enforcing parenting plan child custody agreement under WA state law

custody agreement: we have joint custody,although boys are to remain with me,and durning school year.My son wanted to live with his dad and go to school out there,well,he did poorly in school,and missed many days of school.So i told my ex he is not going to school out there this year,and he refuses to bring him back over here.Also there was no modification done on the custody papers,so he said i willfully abandon him.what are my rights? Also i legally go by those existing papers.Please Help Me... - Is this your question? Add additional information