Hi Pete!
I'm also from England and am planning a road trip that includes quite a bit of what you're doing. As I say, I haven't been yet but I'll share a couple of things I have found that your son might enjoy.
In Vegas we're planning on hitting up cowabunga waterpark to take a break from the drinking/gambling side, and it looks awesome so could be great for you and your son.
Just outside of San Francisco we're staying in Santa Cruz, it has a great boardwalk and fun fair on the beach which you might both love!
I'm still planning my own trip but if I come across anything else I'll come back to this post.
I hope you have a great time!