Just a question…
My husband does have full legal custody of his son but his son has been in NC for more than 6 months
Is it a legal move to get the son back to WA state and if we do can we keep him and not send him back without the mom saying we did parental kidnapping? And make her go to custody court to to gain any custody

It does sound like there is more to the story than the synopsis you gave, so it is difficult to give a complete answer, but if a child custody action was previously filed in Washington and there is a court order that came out of that action, jurisdiction should still be in WA unless the other party has filed a motion to move it and that motion has been granted. If moving the child back to WA would be consistent with the court order, then you should be able to do it without facing any criminal issues.