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I realise your son and you already have your access to BASIC, but I started with the Acorn Electron (the BBC's little brother) and more recently, when I wanted to get in to hobbiest game making, I picked up Dark BASIC.
There's a downloadable free trial, it's very predictable BASIC, and I believe they try to give discounts to schools. Oh, and should the user turn to liking to make games, then it's mind blowingly simple to put graphics on the screen to keep people encouraged quicker.
And then, if they want to go on to C++ without leaving the trainer wheels completely, there's Dark SDK - a set of Dark Basic libraries.
I'm happy with it, and I'm surprised you didn't find it in your searching. There's quite the active community around it too!
http://darkbasicpro.thegamecreators.com/
Good luck to you and your son in the future!