Is the goal to learn to program, or just get his idea working? If programming, I would nix the idea of a 3d game, and get him to do the concept as a 2d game of

Is the goal to learn to program, or just get his idea working? If programming, I would nix the idea of a 3d game, and get him to do the concept as a 2d game of some kind, eg a platformer, or top-down, in Scratch. Scratch is quite limited (IMHO wrongly, and in the wrong ways), and you need all kinds of hacks to get some things to work, but sometimes they make things quite easy, for example, you can make a 'scrolling platformer/tilemap' type game without having to code a tilemap, by setting a huge background image, and scrolling it around. Moving to 3d, there's either a huge jump in conceptual complexity (3d math, etc), and/or a load of learning the tool - eg unity, godot, roblox, etc, a lot of the work is operating the 'studio' and 3d design parts; actual programming can sometimes be just a small part of the project. On the programming track, as a next step from Scratch, I would (and have!) choose lua/love2d, or even fennel/love2d. Love2d is a simple library/framework, and Lua is much simpler than python, and you will (are forced to) learn the basics, for example there's no built-in object-orientation, you make it yourself, so you learn how such systems actually work.