The big draw for him might be Minetests moddability. All of a games content is in .lua files and it can be modified with just a basic text editor. An easy thing to do would be to change the mining time of a pickaxe or change the number of planks you get from a wood block. If he really gets into it, it's possible to create a game from the ground up without relying on the base Minetest Game.
Best bet would be to set him up with the Minecraft clone game. This can be done by going to the Content tab in the menu. Select Browse Online Content. On the second page is Mineclone2.