I know, isn't it weird? Southern California was a famous global tourist destination for decades before Disneyland. Tens of millions of tourists visit California from around the world every year, and a majority of them never step foot in Disneyland. But then I imagine there are people who say
"What is there to do in London for a day or two?" or
"Does New York City have a fun thing we should see?" or
"Is there anything of interest in Italy?"
I think it has to do with the fact that Disneyland is just one of a few hundred shining jewels in the Southern California crown brimming with manmade and natural wonders. But when you are in Orlando by comparison, the only thing there is swamps, strip malls, corporate chain restaurants, and mega theme parks.
I'll give it a shot, but the official Explore Southern California thread is your best resource.
Does your son like cars? The Petersen Automotive Museum is one of the world's finest. On museum row in west LA.
https://www.petersen.org/visit
Does your son like astronomy? The Griffith Observatory and its exhibits and planetarium shows (plus the view of LA off the terraces, or of Mars through the telescope) are awe inspiring.
http://www.griffithobservatory.org/
Art and architecture? The Getty is one of the planet's most lavish and expansive collections of fine arts and architecture, (plus a nice view of Bel Air and Century City). Your entrance is free thanks to Getty Oil and a lot of dead dinosaurs.
http://www.getty.edu/visit/center/?hp-2nd-level-nav=1
Speaking of dinosaurs, does he like natural history, prehistoric beasts, etc.? The famous La Brea Tarpits and attached natural history museum on LA's museum row still thrill, ten thousand years later.
http://www.tarpits.org/visit
Was he into Hot Wheels or model trains as a kid? Next door to the Tar Pits is the sprawling Los Angeles County Museum of Art (LACMA). Just standing and watching the amazing
Metropolis II kinetic sculpture they have is worth the price of admission.
Etc., etc., etc.