This seems kind of strange and poorly thought out. Your nephew is going to Southern Ca. and wants to see something that isn't in Southern Ca. Something that involves a 6 hour drive each way. That is like going to Palm Desert and wanting to visit Sedona, or Las Vegas, or Tucson, Albuquerque, Tijuana, Rosarito Beach, Cabo San Lucas, etc. All of those are less than 6 hours away. If your nephew wants to go to Sequoia or the Redwoods, perhaps schedule a second - - separate - - trip or fly there. If I go to England, I am not trying to fit in a side-trip to Paris (BTW, the side trip to Paris is less than 6 hours). Or vice-versa - - if I fly to Paris, I am not planning a side trip to see Big Ben.
I'd suggest that he (the nephew) consider checking out some of the local to Palm Desert attractions such as the water flowing out of the ground (via the San Andreas Fault) in Thousand Palms (an oasis). Or hike the Joshua tree National Park where there are lots of abandoned gold mines, lots of impressive rock features, a view of the San Andreas fault, etc. Or hike the Bump-and-grind trail. Also, he might find the tram up to Mt. San Jacinto to be interesting.