I wrote a book (free minimum price) to teach my son programming, starting in Terminal, and using Ruby.

https://leanpub.com/programmingforkids

It's currently Mac-only, because of the `say` command, and so that I could write instructions for just one platform. I have ideas about how I'm going to fix that, but that won't be in the near future.

My son was about the same age when I started writing it, but the book scales nicely for any 9-14 year old, I think. Please feel free to give your son a copy, and let me know if it's helpful.