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I would suggest starting by finding a competent WA attorney to file for a modification of custody. It certainly sounds like your son has good grounds for a modification and potentially even the ex's mother would agree. Either way, your son will have to go to WA and set the process in motion by filing for the modification. He should also consider calling Child Protective Services in that area and having them take a report. It could be helpful to him in the custody case.
That all depends on what your son agreed to at the time the custody order was established and also whether or not he wishes to assert his right to custody now. He certainly will have priority over the maternal grandparents, assuming he is a fit parent himself.
I think that it is unlikely that she "signed over" legal custody to the grandparents. She may have signed an authorization giving them authority over certain decisions, but to change legal custody to a non-parent (guardianship) would have triggered notification to the father, which you're stating was never received.
What can you do? Have your son go and assert his legal right to custody as the father of the child. It will be a process but if the mother is as unfit as you state it should have a happy ending for you and your son.
Yes - it's possible you could pay an "attorney service" (they are services that handle courier, copying, service, etc.) and have them copy the court file and send it to you where you live. Then you have the complete record.
Just go online and find an attorney service that works at the courthouse in that jurisdiction and give them a call. There will be a fee but it's probably the easiest way to get the records other than hoing to the courthouse yourself.
What I was saying was that you could walk in and request copies at the courthouse yourself (if you were there), but since you are far away you might want to call and see if a service could do it for you.
And yes - if something was filed in the case it should be part of the public record court file.