Knowing you and your little, I would steer away from Lego Mindstorm for now. I feel you are correct in thinking it will become a paper weight. The guided projects are easy enough to follow, but there is a pretty big leap between following the guided projects and building your own functional projects. It is really more of a Late Middle School to Early High School platform. You want him to have a group a friends or a school class that is teaching the use of the Mindstorm kits before he will get the value out of them.
I was introduced to Mindstorm in Middle School many years ago. The teacher at the time had no programming experience but, was tasked with teaching us about computers and technology. By the 4th guided project, she was out of her depths and the class switched from technology inspire projects to teaching typing. As I still hunt and peck when I type, I would say that she wasn’t that good at teaching that either.
So, unless you are going to learn the projects and guide you short stack through them, I would suggest punt this down the road till he has other friends doing this or a school class teaching this. Not saying he couldn’t learn it, just that your about to give him Legos and a 150 page manual on how to use them. How far do you think he will go with that?