I think you already have everything you need, but here's a copy of the Consent for a Minor child to travel (which you probably have):
CONSENT FOR MINOR CHILDREN TO TRAVEL
I (we):_____________________________________________ _________________
authorize my/our minor child(ren):_______________________________________ ____
to travel to:_________________________________ on:_________________________
aboard Airline/Flight Number:________________________________________
and/or Cruise Ship:_____________________________________________ with:
__________________________________________________ __________. Their expected
date of return is _______________________.
In addition, I (we) authorize:______________________________________ to consent
to any necessary routine or emergency medical treatment during the aforementioned trip.
Sworn to and signed before me, a Notary Public,
this _______ day of _______________, 20____
Notary Public Signature and Seal
The only other thing I can think of is a credit card or cash for your son's friend. Otherwise, I think you're covered! Have a great cruise, and what a lucky friend!