I wonder if starting with C/C++ is a good choice for kids at this age?
Maybe python or Javascript would be better (the later with the advantage of being available in any web browser). In Chrome just hit CTRL + SHIFT + J and you have a Javascript console. There are quite a few tutorials available for free in the web for both languages. Plus your son could easily share with friends the pages he created.
Even though I like the Pi, it has a few disadvantages that it requires a keyboard + LCD monitor (so it is not really 'mobile' and if you add a decent keyboard plus a good monitor you are past the price of a ChromeBook that is truly mobile).
What is attractive is the price around 60-80 per kit (*if you already have a keyboard/monitor*) and being a Quadcore ARM processor you can actually use it as a 'desktop', assuming that your kid is comfortable with Linux. This video is quite nice in the sense of showing Rpi3 as a desktop:
https://www.youtube.com/watch?v=oq0qE9KYtEgMy daughter was using Ubuntu at age of 5, ChromeOS at age of 11 and Window/OSX now at 15. The earlier you expose kids to different operating systems, the easier is for them to adjust and not being addicted to MS Windows.
:-)