I would recommend flying to Vegas and then driving from there to Death Valley, from Death Valley to Mammoth the road is phenomenal!!!
Then Tioga Pass into Yosemite for more insane scenery. I would break this journey up over 2/3 days though or it is a long time in the car.
Approx 4 hours from Vegas to Death Valley, then plenty to do there for the rest of the day. There is accommodation in Death Valley but we drove out over the mountains at sunset and it was unbelievably beautiful. There are a few small towns before you head up into the mountains you could stay in or another 2/3 hours gets you to Mammoth Mountain where there are some lovely hotels with pools etc.
Then I would spend another full day driving from Mammoth to Yosemite (book your accom here as early as possible it gets full!). Mono Lake is a fascinating stop and the Tioga Road is insane. Plenty of scenic stops along the way down to Yosemite Valley- we stopped for a picnic and a paddle by a lake and some amazing viewpoints.
Spend several days in Yosemite, it’s phenomenal- drive up Glacier Point, see the Giant Sequoias, Yosemite Valley, if you think son will be up for a hike, you can do the start of the hike up Half Dome as far as Emerald Lake for stunning waterfalls.
The drive from here to Monterey is fairly boring, approx 4 hours, but it’s worth it. Monterey has an incredible aquarium, whale watching trips, gorgeous white sandy beaches, Carmel worth a wander, 17 Mile Drive, probably another few days there.
We only had 2 weeks so went from here on to San Fran for 4 days then flew back to Vegas for another 3 to fly home from there, but if you have 3 weeks, I would drive to San Fran for a few days and then head down the Pacific Coast Highway to San Diego. Loads of amazing stops so you could spend a few days here.
In San Diego there is the Zoo and then theme parks definitely worth a stop in LA- I preferred DisneyLand to DisneyWorld and Universal has the genuine studio tour- beware though, LA is huge and it’s literally hours drive between the 2. Either stay central and take day trips out to each or a night at Universal and then on to Disney!