I agree a Volvo 240 would be good for a young guy to learn about the really simple mechanics of what goes on, but a 240 specifically, no. The 740 or 940 have the same engine and gearbox etc, and are readily more available (more elbow space under the bonnet too) and not subject to the massive increase in price. Also, much more room in the boot for camping in the back (can fit Ikea double mattress for extra comfort, I camped in the back of mine for 2 weeks touring Germany)
As for learning mechanics, with the redblock engines (240, 740, 940) you'll top out pretty quickly to be honest. I bought a 940 which was going to be scrapped to fix up - took me about 2 weeks, although I have a lot of experience. So your son is gonna know the ins-and-outs of the car in maybe a year or two. Plus, they don't make cars like that anymore.
I'd say, yeah, go for it, but don't expect the puzzle to last that long.
Example 740
As for MPH - I get 16mpg in my 850 T5, 18 in my 960 and 20 in my 940, all automatics. Got a friend who has a new Mercedes diesel bluetec, advertised at 75-80mpg combined or something, he gets about 38 in reality, so MPG is a load of crap anyway.
240/740/940 has plenty space and capacity for an electric motor conversion and a large battery pack in the future as well