Ditto the K-1000. All manual, rugged as a tank. The match needle metering is more instructive than LEDs. Repair parts are plentiful if needed. It uses any K-mount lens, which for a kid can be picked up very cheaply. I have a big caveat, though.(posted 8277 days ago)I have recently switched from an ME Super to the Pentax KX, which has more manual features I wanted. I let my 12 year old nephew use the ME Super for a day of shooting at Colonial Williamsburg and he managed to clobber the winding mechanism. this was my own fault, for not thinking it through. There is a lot to do when you have to set exposure and focus and compose at the same time. Combine that with all the other visual stimulation going on, and I think he was simply overwhelmed by the complexity.
I read a few articles since about getting kids started in photography, and they seem to concur with each other that simpler is better. I started with a Kodak Baby Brownie (127 B&W film). Maybe a basic point and shoot would get an 8-year-old faster positive feedback in terms of results. Then, as he sees you using manual features, he might want to get more versatility.