Do we have to file for legal guardianship throught the courts if all parties agree?

My nephew that live in Wisconsin (I'm in California) wants to come out and live with us. They came for the summer last year, and he loved it, and loved how he has so much family and cousins out here. Where they live they have no family, their aunt lived out there, but has since moved back out here. His mother and father both agree to this, and are willingly sending him to us. There is no abuse going on, theirs no drugs involved, he just wants to live in California where all of our family is. The rest of their family, mom, dad, brother, and sister do not want to move back here, they're happy where they are. So my question is do we have to go through the courts? Or can we just do some type of notarized letter stating that they give us full guardianship? In order to file for guardianship is going to be roughly about $600 without an attorney, plus they'll have to fly out for the court hearing. We really don't want to have to pay so much, if we don't have to. Our main concern is for health insurance, in order to add him to my husbands insurance I believe we have to have guardianship? Any thoughts?