Hi, you may want to also explore the idea of local museums - they often have great activities or interactive exhibits and some even do birthdays. It's a fun, unique, and sometimes even educational (the horror) way of celebrating a birthday with a small group.
Here are some suggestions:
-Kidspace Children's Museum in Pasadena (at the Rosebowl) - you can also have a big picnic afterwards in the park areas
-Discovery Science Museum and see the Dinosaur Exhibit
-Museum of Natural History
-American Heritage Museum
-Air and Space Museum
If he enjoyed the movie "Night at the museum" (particularly the second one), he and his friends might enjoy traveling to one (or multiple) spots that was used in the film (the plot took place in NY and DC but many of the scenes were actually filmed here in LA).
I remember also seeing a great place for a birthday out in Irvine, it's an outdoor mini park that has go karting, batting cages, mini golf and an arcade that had really reasonable prices.
Perhaps he and his friends may like something more active like hiking to a waterfall. Stuyvesant Falls in Arcadia is a great adventure to go on with younger children and ends with quite a large spectacle.
Hope this helps.