Mott is a member institution of the Children's Oncology Group, so you are indeed in the best hands. As a COG member, your son's physicians will benefit from collaboration and data sharing from physicians across the network.
Not sure whether proton radiation (or radiation generally) has been mentioned as an additional treatment after chemo has been concluded. If so, I do not believe that there are any proton radiation centers in Michigan. If you end up coming to MD Anderson in Houston for proton radiation therapy, know that our Foundation can assist you with free housing and other support services.
Much love to you and your son.