I've heard of a number of kid friendly games, but counterintuitively they usually use d6s.
Maybe you could steal some ideas from Dungeon World? I've heard that kids handle it well. Risus would also be a good candidate for modification.
As far as world building, in the traditional sense, I've heard of some really great results from just asking the child "what do you think is there?" when they come to something new. Then play off of that. A lot of times it ends up giving them what they want and often is hilarious which makes the game fun for the adult.
In the minecraft sense of world building, You'd need a simple way of crafting that would feed into what your son is used to. Ideally that might also teach him something about making things in the real world as an added bonus. I use game crafting systems with my boy to teach him material sciences.
To keep it simple for a young lad, (How old is master Lang?) Have him, in the game, take two things. He can put them together and either Heat them up (need a hot enough fire, lava is usually the hottest thing a kid can imagine so it's a crafting item), Dissolve them (water works for dirt, acid works for harder things like stone and metal), or Shape which may require tools like hammers or knives but in the case of dissolved dirt only requires hands.
Whenever he tries to make something you suggest the different methods and then try and work out what happens in the result. You have to stretch the things he could actually do, and he'll probably need a good supply of lava to keep crafting.