I was going to suggest something like Kaboom.js for simple game development, but their site is currently down...

Some high-level ideas here:

There's lots of opportunities in programming to make mistakes, or where they're faced with big open-ended problems that a programmer can build an entire career in dealing with in themselves. Being aware of what resources you need to look for and learn from is one of the first steps to getting what you want out of your interactions with those areas. If he persists in learning programming, he will soon run into these areas, and how well he handles them will play a large role in determining how successful he is as a programmer. A small number of examples of different domains that have unexpected nuances which suddenly become important to a new programmer, and catch them by surprise:

Internationalization Password handling Interacting with the host operating system and filesystem Interacting with programmers from groups with different social norms and standards than they're used to - this is partly solved by READMEs and project documentation, but only partly. Social skills are important. Interacting with end users Programming languages often have weird corners to them which can waste a lot of time, cause a lot of confusion, and cause bugs. Looking at C and C++ especially, but no programming language is blameless. He'll have to test his code. If he needs to do involved tests on his code, which he might eventually, there are a few areas of testing code are virtualization, unit tests, and fuzz testing. These all take significant effort to learn how to do. Ethics, legal issues, and politics. GUI programming

There are plenty more areas, of course. I'm not saying this to discourage you or your son - learning about these sorts of things can help your son grow as a programmer and maybe even as a person if he's fortunate. But my biggest piece of advice is to be very careful where you put your time and attention on, in programming, and probably in life in general. There's no limit to the time you can waste on being foolish, except the limits you have for yourself.

My second biggest piece of advice is that maybe he should switch to using Linux and Free and Open Source Software at some point. It's very convenient from the perspective of a programmer who's very motivated by being engaged with a community or by benefiting others - he can learn about a community he's likely to want to engage with in the future, he'll be more able to change aspects of his computer to suit his needs or desires, and honestly, my experience as a Linux user who's learning programming is that learning how to use Mac and Windows feels more like work than learning Linux does. Learning how to use Linux feels like learning how to program.

Maybe you would want to introduce him to Godot and Blender, since they're FOSS software, and less at risk of weird decisions being made with them like happened with Unity, where companies pulled a very weird and developer- and user-hostile and security-hostile decision out of nowhere.

Try a more advanced node based programming language that aligns more with a specific language. Something like NodeRed for Java...

...or maybe he's ready to try game development on something like Godot, which has a language a lot like python.

Python is apparently a good first language to learn.

Alice is a pretty good 3D programming environment aimed at kids, with little programming blocks to snap together.

You might want to try going back into the archives and pulling out something like MS-DOS and QBasic, or Logo. You can find a good tutorial in book form, and you can get a system that was designed to be programmed offline, with things like local help in the editor instead of behind a Google search, so it should be 100% safe to leave the kid alone with the machine.

When I was a kid, this book was fun

https://archive.org/details/vgmuseum_usborne-hayes_introcompprog

All the others have great suggestions but I'd just like to add there many open source projects out there and given example code of how something is done and letting him play with it can be tremendously helpful. For example finding some simple 3d programs and lettting him change the source code to do whatever he wants or just giving him a template like https://editor.p5js.org/1alimaze/sketches/IJpxIEME8 which I made when I was first learning and letting him play with the values and add more objects.

A couple of things on resources: he has expressed interest in 3D worlds and I noticed comments on engines, but wonder if that’s too advanced?

I most certainly wouldn't start out programming with / in 3d worlds.

Drawing 2D shapes seems like a good way to spark interest though, with immediate visual feedback.

minecraft can work really well for learning how tp program, when I was 9 that is how I started, unfortunately its been a long time since I took that course but if you are willing to do the research there should be a some for server programming and java modding at least that is what I took over 11 years ago at this point, and maybe javascript for gametest framework for bedrock edition. there is also minetest which is a little better for learning but isn't minecraft so is a little bit more annoying but lets you modify basically anything you want which is very fun. also a great way to learn boolean algebra/circuit design in minecraft/minetest, redstone can be used to make micro computers like calculators, fun and can be applicable to making games in minecraft which makes it more engaging as well. also the minecraft commands can be put into a mcfunction file as you can imagine this can be generated by outputting text to the file, this is great because you learn file io and a programming language and mostly he would be using math to generate things so teaches math, its limited but that is something he would find fun that is super easy and can be done in basically any language.

redstone/mcfunctions I personally like this approach on top of the course because it gives him something useful so should hopefully encourage him to play and program after he finishes lessons and etc...

IMO set him up with some of the modern engines and tutorials... Godot 4, Unreal etc...

actually if you are looking for some stuff to have ready for him.

https://www.humblebundle.com/books/game-programming-with-c-unreal-packt-books?hmb_source=&hmb_medium=product_tile&hmb_campaign=mosaic_section_1_layout_index_2_layout_type_threes_tile_index_1_c_gameprogrammingwithcunrealpackt_bookbundle

If he has access to youtube there's also lots of really good tutorials etc... out there. Heartbeast has some awesome ones for godot etc...

a linux web server on an old PC https://www.linuxbabe.com/ubuntu/install-lamp-stack-ubuntu-20-04-server-desktop , a tutorial https://www.w3schools.com/ , and a text editor https://notepad-plus-plus.org/ is all you need to get started.

Ground him into a room until he learns to make a game with only assembly, give him a Tandy to figure it out