am trying to determine if I need to carry written
permission from his parents to travel with him

I'm not sure if you'll really need that for domestic travel, but forms
like that are usually called something like "Temporary Guardianship
Authorization". The parents can fill it out and then get it notarized.

Here are some examples:
http://www.theaterofthesea.com/autho..._care_form.htm
http://www.free-legal-document.com/f...ship-form.html

Also the reference desk at your public library might have some on file
that you could Xerox, or maybe your local probate court.

James