My younger brother is 16, and last time we went he was almost 14.
I remember he liked:
Buzz Lightyear's Space Ranger Spin (MK)
DINOSAUR ride (MK)
Splash Mountain (MK)
the Jungle Cruise (MK)
Spaceship Earth (EP)
Haunted Mansion (MK)
the World Showcase (EP)
The Great Movie Ride (DHS)
Mission: SPACE (EP)
POTC (MK)
Some other things he might like are: Soarin' (EP), Rockin' Roller Coaster (DHS), Test Track (EP), Lights Motors Action Extreme Stunt Show (DHS), BTMR (MK), Studio Backlot Tour (DHS).
It really depends on what your son likes. My brother really doesn't like the "kiddie" stuff like Peter Pan's Flight (which is my favorite ride, and MK is my favorite kingdom). He's still a big kid though, and he relaxes once he actually gets to WDW, and he has a blast with us! There is stuff your son can enjoy in each kingdom.
Plus, remind him off all the awesome foods to try in EPCOT and turkey legs in MK!!