If he is interested in Casa so much, I think Fes might grab him. The old city is killer, but as you said, its a good launching base to boot. Buses to and from the cities you mentioned are easy and it sounds like you have a good understanding of the country. Just spending a day in Fes, a day in Volubilis, and a day in Meknes will soak up your time there, not to mention travel to and from Marrakech (you might want to get there the night before your flight?)

What about seeing the market in Azrou thats on Tuesdays? Its not only quite the experience, but it allows you to see/understand Berber culture. An easy day trip from Fes or combined with a trip to Meknes. Have him get a hair cut there

My rec would be to see Marrakech, Fes, Volubilis, Azrou market, Meknes (and Berber walled old city). That would easily fill up a week itinerary and give you a great feel of the many shades of Morocco. Berber, French, Arab, Old, New, Desert, Mountain.