Your son is so lucky to have a mom like you!!! It WILL be a special trip because you'll be making memories together. Even if you get sidetracked and don't make it to the coast, whatever you do will be terrific. You're the greatest!
Try to find some direct route from A to B that don't involve the interstates. You'll see a lot more of our beautiful country. With a tent you'll have many choices in the national parks, state parks and national forests as you travel. Every campground does not reserve 100% of the sites. Pull in by early afternoon (morning even better) and you'll get a site - even in Yellowstone where there are many non-reservable campgrounds. Try to work in at least a night in Grand Teton Nat'l Park just south of Yellowstone. It's a beauty. Colter Bay and Gros Ventre campgrounds don't accept reservations and there are 300 sites in each.
If you want to drive the California coast a little, try to route yourself to Hwy 1 above San Francisco. Perhaps start at Bodega Bay (a few nice county parks right on the water) and work your way north to Fort Bragg. It's an awesome wild coastline with plenty of campgrounds, and a smidgen fewer people than further south. Then head inland to Hwy 101 and if you have time before turning around for home, spend a couple nights around Redcrest, CA and tour the Avenue of the Giants (redwoods). It's easy to access and there are many great, easy and fast trails to do just off Hwy 101. Please report back and tell us how it went...safe travels!