Where are you starting /ending from?
Sequoia /Kings Canyon from the East is either Whitney or access to the PCT from a few high trailheads. The big trees and caves (more caves than any other CA park after Lava Beds) are accessed from the West. Also, although S/KC look close to Yosemite, there are a lot of twisty road (Hwy 49) or backtracking to Hwy 99 involved.
I'd couple Yosemite with a variety of things to the east, especially if your are approaching from that direction.
The State Historical Park and ghost town of Bodie,
Mono Lake (visitor's center in Lee Vining)
fissures in Black's Point on the north shore of Mono Lake.
Tufa formation on the south shore of Mono Lake.
Devil's Post pile
Bristlecone Pine NF on the east side of Owen's Valley with the world's oldest trees.
Natural hot springs, one developed and many other lesser developed ones east of 395.
All of the above have moderate to high altitude, so if you do those as you approach your Yosemite hike, it will help both of you acclimatize to the higher elevations.
If you are thinking of the Grand Canyon on the way, it is not a good season to go the River and back and it sounds like he hasn't proven himself as a hiker for that level of day hike. Therefore, if not going to the River, you could go to the North Rim which is far less crowded, cooler and more forested. Also, that puts you closer to Zion and Bryce. Leave the impressive, expansive views from the South Rim for another day. Maybe when he's strong enough for a RIm-River-Rim hike (but not in the summer). I did it 2 years ago when my son was 11 in late May. We got lucky and it only go to 89 in the Inner Canyon that day.
If you go to GCNP on the way to Yosemite, you WILL pass by Hoover Dam which is worth a tour. You'll be close to Death Valley, but that's not a good time of year for doing much more than looking out the window. Maybe overnight outside DV and go through in the early morning, a take a short hike shortly after first light, or even before.
I spent 8 days with my son and hiked the Grand Canyon, toured Death Valley, started on the Whitney Trail (too much snow), did some Yosemite hikes (too much snow for Half Dome), Sequoia, Kings Canyon, a few things in the Central Valley, And various LA attractions. But he's quite the hiker for his age and we both road trip together very well (and very quickly). It was a little heavy on the driving because Tioga Pass into Yosemite was closed due to heavy snow until far later than usual. (The next year, it was open quite early.)