domestic travel w/14 yo

I will be traveling next summer with my grandson, who will by then be
14 yo. We are going from OR to DC, NYC, Phil., not leaving the
country. Have checked state dept and several airline sites for info
needed ID, travel permission docs, medical permission docs, etc.
Obviously, once we have airline tix I will check directly with the
airline, but am trying to determine if I need to carry written
permission from his parents to travel with him. I also believe I
should have an authorization to provide medical consent. I have
looked for info and forms online, but everything I see relates to
international requirements.

Years ago when my son was this age I remember haing some preprinted
forms which I provided to his grandparents. Any ideas?