My suggestion for a good, solid Mac for vintage gaming with lots of future possibilities (should he get "the bug") would be a Macintosh IIci. It's plenty expandable (3 nubus, PDS slot, up to 128MBs of RAM), reasonably fast (68030), can be had for fairly cheap (under $50), fairly easy to find, can use both 24-bit and 32-bit memory mode (important for compatibility), and can run everything from System 6 to 7.6 in its stock configuration.
My alternate suggestion would be a Quadra 605 or LC III. Fast CPU, good amount of memory, and you can use an Apple IIe card in it should he also want to experiment with Apple II stuff (though a real Apple II might cheaper at this point.)