In my experience, players who are into this are really into it and players who aren't, really aren't. Therefore, bluebooking - these days, probably mostly by e-mail - is the way to go. Provide the player with a rule-set (it really doesn't matter which one as long as you both can work with it) and let him go to town. As long as he doesn't want to do anything campaign-breaky with it ("Can I have a gold mine? How about I build a Wizard's Only subdivision and as part of the rent I get magic items! My neighbors like me so much they vote me in as Supreme Dictator for Life!"), and as long as he doesn't pile up your workload, let him do whatever he wants. Take care of details like income vs. expenses, time required for maintenance, etc., in some mutually acceptable, low-labor fashion; random generation tables he can roll himself are good. If it becomes inconvenient, make it either a drag on the game, an adventure hook, or both. ("Oh, sorry, you were off-plane for a year and never communicated home, so your steward had you declared dead and took over your business.")