There are more attractions for your son to enjoy at Universal Studios versus Islands due to height restrictions.
Depends on how tall the son is, and how adventurous he is. My son preferred IOA over USF for one simple reason: Spiderman
This is from the official website:
NOTE: Children under 48" (1.22 meters) must be accompanied by an adult at all times and on all attractions.
Revenge of the MummySM- Must be at least 48"
Jimmy Neutron's Nicktoon Blast- Must be at least 40" (persons under 40" can experience this attraction from an adjacent stationary location.)
Back To The Future The Ride® - Must be at least 40"
MEN IN BLACK Alien Attack - Must be at least 42"
Woody Woodpecker's Nuthouse Coaster® - Must be at least 36"
Jurassic Park River Adventure® - Must be at least 42"
Pteranodon Flyers® - Must be at least 36" (36" to 56" must be accompanied by an adult)
Dueling Dragons® - Must be at least 54"
Flying Unicorn® - Must be at least 36"
Dudley Do-Right's Ripsaw Falls® - Must be at least 44"
Popeye & Bluto's Bilge-Rat Barges® - Must be at least 42"
Incredible Hulk Coaster® - Must be at least 54"
The Amazing Adventures of Spider-Man® - Must be at least 40"
Doctor Doom's Fearfall® - Must be at least 52"
The High in the Sky Seuss Trolley Train Ride! - Must be 34". (Children 34" to 48" must be accompanied by an adult.)
There is also answers to this and many other questions on this very board:
http://disboards.com/showthread.php?t=1353733