I gather from poking around online that I could probably figure out how to use Unity to make a very basic game prototype, even with 3d elements. Neither my son nor I can art worth shit, but I was wondering if there might be ready-made public domain 3d assets he could take advantage of, kind of like public domain images but in 3d. Are there such things? Preferably with a variety of like, monsters and ninjas and things available. 
The Unity store has several free assets, mostly form tutorials, but also from third parties. You can also purchase assets for prototyping, most of which are reasonably priced.
You can even use primitives - shapes like cubes and spheres, etc as stand in’s to start, and then worry about nicer stand ins later.
Here’s a list, I’m not sure if it’s up to date:
There are a number of websites out there that also offer free 3d models and 2d sprites for game prototyping, just looking at google I found this:
Finally, blender works super well with Unity (you can literally drag an drop the files in your unity project and things usually just work), so you may want ot give some unity tutorials a shot.
I can’t art either, but I’ve managed to build pretty cool little space ships, just messing aorund with the editor.
There’s free Unity Assets available for what you’re looking for. A number of people actually release “full” games with stock assets (derisively called “Asset Flips” among gamers) but nothing wrong with going cheap to see if his idea is a good one.