I don't know how old your son is, but I think he'd be better served by learning something relevant and extensible, than some custom language intended to him from scary programming. The Python and editor idea was good, though it doesn't necessarily have to be Python.

He's got you there so if he wanders into some "dark corner" of the language it's not like he's screwed and without help. That was my major impediment to learning programming as a child, not having someone there to explain things. I was 6 years old and got stuck on the concept of an array and my progress ground a halt. Later on I found a friend who could explain harder things like structs and enums and this made programming much easier.

So, just show him tools and be there when he has questions is my best advice.