Have a look at the many online tutorials. Some links are...
http://www.programmingtutorials.com/main.asp
http://alslinks.virtualave.net/
A Google search for 'programming tutorial' will find over 20000 refs. Some of these have fully working example games programs so that your son can see methods of design.
I wasn't too impressed with the keyboard on the zx80 - learned to type on OLD remington machines - so I opted for the Commodore range, VIC20 & C64, but all of the machines could be fun to learn on. Given the small amount of memory available you had to be careful with your programming style. I got fed up and switched to assembler.
I hope you find something useful in here.