How do I grant my brother in the state of Washington guardianship of my 17 year old son, I am in Arizona?

Both parents can grant a family member a Temporary Power of Attorney, revocable at will, which will allow the person to enroll their child in school, take him to the doctor, etc.  Any Washington family law attorney should have the form.  Whether the Washington colllege will consider this and residency enough to establish domicile for in-state tuition is another question.