under WA state law can ex-wife try to regain custody of son after walking out on us

Either party has a right to Petition the court to modify a PP. The moving party must show that there is a substantial change of circumstances that has occurred since the last order was entered whcih justify the modification. There's not a lot u can do to stop him if he wants to do that, but he will probably have an uphill battle. The best thing you can do is abide by the provisions of the PP to the letter and require your ex to do the same. A 15 year old child does not have the right to choose where he wants to live. A court may coinsider his wishes if a modification is filed, but the court is not required to grant that wish. The court's first obligation is to ensure that the best interests of the child are being serviced by the parenting plan.