Thank you everyone for all of your great suggestions. It seems the consensus is to try out Tinkercad first so that's what I will do.

@kyle Tinkercad is a great way to introduce the topic. I use it for my projects.

@kyle I started my design journey using Tinkercad. Web based, also programmable. Moved onto Fusion 360 after that. Keeping in mind that I am good at engineering type design. My skills at artistic design are still poor/non-existent.
Tinkercad can be easily started by dragging and dropping shapes, resizing them, combining and modifying. All in the browser with keyboard and mouse.
www.tinkercad.com

@kyle my dad uses fusion360 but that's hardly Foss and I hate it... FreeCAD is the best thing, but learning to use it is a big pain. Maybe you can show him how to use the part design workbench - sketch and extrude, that gets you far. But that thing is completely filled with footguns no matter what.

This can also make simple models right in a slicer from primitive shapes

@kyle I second the use of Tinkercad as an introduction. If your son shows interest and commitment, I would not hesitate to move him to a product like Fusion 360. Tinkercad can create a lot of "bad habits" if someone wants to move to a more functional product.

@kyle Sketchup 2017 (non cloud) still available if you hunt for it.

@kyle Solvespace is easy to use and it's FOSS. It's much easier to get into than FreeCAD but it's a proper 3D CAD tool nevertheless.

https://solvespace.com/index.pl

@kyle Take a look at LibreCAD https://librecad.org/

And of course, Blender. Believe it or not, sculpting in Blender has a lot of tools to make symmetry pretty easy now, so you don't need to be an expert sculptor to produce something interesting.

@kyle While there's no user friendly CAD free software that I know of. There's this
https://github.com/nkallen/plasticity

It dynamically links to a proprietary CAD engine and is an electron application, I think it might be fit your needs at the moment.

I personally believe it's possible to replace the proprietary CAD engine with a free software one.

@kyle Hey, regarding your post i would recommend tinkercad, a free web based interactive 3D-Designer that does not require a lot of CAD-Skills and is really easy and interactive to use. i would imagine a 10 year old could use it right away, playfully learning 3d-shapes and modeling with different kinds of shapes and figures.
https://www.tinkercad.com

Please keep me posted if it was helpful :) Thanks and good luck! :)

@kyle FreeCAD is definitely not for the faint of heart. I'm still climbing that hill myself. The good news is there's a huge user community around it with lots of text and video documentation and demonstrations on how to accomplish lots of tasks. Good luck!

@kyle We used https://www.tinkercad.com/ for an outreach project some time back. I wasn't running that segment of the program, but my colleague who did sang its praises. Online only for better and worse.

@kyle I started with tinkercad. Free, browser based, and pretty simple starting point.

@kyle https://www.tinkercad.com/
It works in a web browser.

@kyle I helped with an after school maker program and the kids there were very quick to pick up tinker cad. It runs in a browser so even chrome books could handle it. The kids were also using touchscreen laptops and making it look very natural to pan and zoom around objects. It’s not full cad but it will teach a lot of concepts and get them 3D printing stuff very quickly.

@kyle Maybe https://www.tinkercad.com ?

@kyle

FreeCad
OpenSCAD
Blender

@kyle Have you looked at TinkerCad? It is web based and pretty easy to use.. Your 10yo will probably pick it up quickly and can move on to something more advanced from there when they are ready