In WA, a person generally becomes a legal adult when the person is 18. When a person is a legal adult is when a person is free to decide with whom to live. Until then, the person is subject to the control of the person's legal parents or the court.
While the court may considered the opinions of an older child as to where the child should live, the court has no duty to abide by the opinions of the child.
You may have a basis to file a petition for third party custody. If the mother has been in and out of jail leaving you to take care of the child for the last four years, the court may well determine that she is an unfit mother.
Unless the grandson's father is dead, the father will need notice of legal action your bring regarding the child. Unless the father is also an unfit parent, he may be getting the grandson if he asks the court to have the son placed with him.
You should review the specific facts with your attorney to find out your legal options.
While a child has the right to move at age 18, there are other options available to you. You may wish to consult with an experienced family law attorney in your area about a "Third Party Custody Action". These are not easy but you may have a reasonable chance of success.
You need to file a petition for non-parental custody right away if you want your grandson to continue to live with you. If you are at all able to afford to have an attorney represent you, it is a very very good idea. If you can't afford an attorney's representation, at least please meet with a good family law attorney to consult with them about your rights and how you go about doing this. Even if you choose not to interact with attorneys, the courthouse probably has resources for your kind of case. File your case before your daughter gets out if at all possible. Best wishes to you and to your grandson.