Lot of comments suggesting children's toys, but I don't think that's the best way to go here.
An old VCR is safe enough. He might cut a finger on a sharp edge but that's ok. Sounds like he's already curious, so let him have at whatever you can scrouge from a junk sale for $5. Cheapest education he'll ever get.
Some other ideas that cost next to nothing and let him explore the open world we live in:
USB Microscope - he'll be able to see the mechanics of texture in silk vs cotton and salt vs sugar vs flour, as well as the 100s of creatures in a drop of puddle water.
Infrared thermometer - highly useful and informative from now into adulthood. "Hey mom the oven temperature is wrong, it says 400 but my thermometer says 380," and "Little sister has a fever, it's 100.5 inside her ear," and "it's 935 degrees in the fireplace, and the windowsill is 41." Wholesome stuff.
A kitchen scale. High quality magnets. Samples of different materials. One of those green plastic sheets that make magnetic field lines visible.
And real tools - whatever he's ready for. In my generation, boys got their first pocketknife at age 6-8. I'm sure he'd appreciate a quality multitool.
If you love the kid, the next time you have to fix something like a lawnmower or a car, do it at his house and both of you will have a fun afternoon in the garage. Bring lemonade to keep the bloodsugar up. He will remember that day fondly for the rest of his life.
tl;dr: skip the toys. show him the real shit.