I'm the new family cruise editor for CruiseMates.com and I queried officials at Disney about your concerns. Here's their reply:
"An 18-year-old can enjoy all the experiences that adults can (spa, adult pool/Castaway Beach, Palo restaurant, nighttime entertainment area, etc.) On the occasion that an 18-year-old is traveling with friends and family that are near in age (16, 17 years), and that guest would prefer to participate in activities with this teen group, we can accommodate that guest in areas such as Common Grounds, our teens only club. We ask, however, that the guest decide between an adult experience or teen experience without switching back and forth between the two during the cruise."
"Also, we are working on trying to gear some of the teens' events to just the older teens. During the seas days, there are pool activities and game show events. When the ship is in Castaway Cay, there is snorkeling and boat rental, beach volleyball, etc. "
I hope this is helpful and that your son finds which activities and age groups suits him best. He may just want to check out the teen club and see if at least he can hook up with other teens his age.
Luisa Frey Gaynor