GNU radio is an awesome signal processing toolkit. You can use it to take signals from a cheap RTLsdr, or even a microphone, and make new virtual instruments, radios, etc. Long ago I was interested in aircraft navigation aids, and was able to throw together a receiver for the local VOR transmitter, which showed me on screen where I was relative to the transmitter.
You could build a sonar system, listen to ultrasound, or almost anything you can imagine, thanks to Moore's law and the massive amount of compute we have these days.
Perhaps it's too simple, but I like https://elenco.com/snapcircuits/
If he likes video games, we’re hosting a big game jam for middle and high schoolers next weekend.
It’s free and nonprofit. There are 60 locations. Maybe one is near you?
I'm a really big fan of the more modern way of doing things.
A lot of old school maker projects are fun, but they're also basically E waste making kits now that phones do almost everything.
Not having a bunch of low tech gadgets laying around is really nice, they can easily become the biggest clutter challenge in a space, the same way paper used to back when more people still used it for things other than books.
Building a small solar energy system is fun and easy, and also practical.
3D printing is fantastic.
Meshtastic has a lot of DIY and learning potential and is like the lite edition of ham radio. What's really cool is that range testing gets you outside.
Maybe design something together with JLCPCB and have it made(Leave off any through hole connectors and solder them yourself for both fun and pretty decent cost savings).
Any solar energy kit you would recommend for a hobbyist without breaking the bank?
The prices change every 5 minutes for sales and such, but there's generally always a ton of 50 cents a watt solar panels on Amazon.
Permanent installs that work unattended are slightly scary and have to be done right so something doesn't leak or start a fire, but the portable folding panels are usually similarly cheap.
You could get one of those plus a charge controller and LiFePo4 for not too much.
Everything still needs to be done carefully though, and unfortunately almost all cheap charge controllers have a ton of idle power draw, they're kinda crappy compared to the tech built into a random cheap solar generator, I don't think you can make anything as nice as a commercial generator for cheap.
Most the DIY education kits for kids type stuff seem pretty crappy too.
An ESP32 module with onboard lithium charging plus a USB-C folding panel has a lot of possibilities though, if you have any projects ideas that could be done with such things.
Take a look at what these guys do:
Modular synthesizer.
8th graders need to make noise.
Good luck.