As always there's a lot of people that are smarter and have more / different experience than me so it would be great to get any other input on OPs question.

I'll give some suggestions, though I must warn that I haven't used these things. All the stuff I grew up with is outdated or cannot be bought any more.

Probably one of the best things you can do is start with a book of projects for kids, see what is most interesting and then acquire the materials and tools to make them. I don't have any that I can recommend off hand - you can probably find a ton of them at the local library and just check them all out (this is what I did growing up). Books are much better than blogposts or other internet resources for a lot of reasons I won't get into right now...

I had a kit similar to this 100 in 1 electronics kit and it was fantastic. Walks you through 100 different electronics circuits and physics experiments. It's not too expensive and is a great way to develop an understanding of electronics, motors, etc.

If he's into robotics there are an enormous amount of really well priced modular robot kits you can find on ebay / amazon / etc. The good ones can be built into tons of different configurations and have programmable microcontrollers that will allow him to get into the programming side. Here's an example of one that looks promising, though I would stress that there are a TON of options. You can probably find a subreddit devoted to these types of things that would give you good advice on the best bang for buck. If he goes this route there's enormous quantities of more advanced components and kits to progress to.

Another fun direction that I put a lot of time into growing up was RC planes. You can build them from scratch and pretty cheap. Once you have the basic motors / controllers you can build entirely new airplanes from foam sheets.

As for tools, a cheap 3D printer + simple cad like Fusion 360 is an incredible never ending source of parts for building things. You can find pre-designed things to download and build or make your own. You can get a decent printer for $100-200.

Hope this helps. I'm glad to hear I've made such an impact. I was hoping to get more people into engineering so this is nice to hear :)