Garage sales are a great source of old computers and appliances, too - usually free. Computers are serious fun to dissect (but they might not go back together so well).
For less supervision-intensive play, if he's not already too old, you can also make a wood block(s) with lots of different sized and types of screws, bolts, and whatever else is within your safety margins.
At our local kids consignment store, I found a number of plastic build-and-take apart toys (trucks, cranes, etc) that could be (dis)assembled with plastic drill/ screwdriver, wrenches and sockets. Some had many different kinds/ sizes of attachments, and 40 or more parts to fit and attach together, so these could get a little more sophisticated.
And of course, there's Meccano, of the traditional ilk as well as the younger kids plastic stuff.
