We spent two weeks in Italy, divided pretty evenly between Venice, Florence, Rome and Naples. Don't skip Venice - it's unique with the canals, the Isle of the Dead, Merona (glass blowing - fascinating). We flew into Venice and out of Naples, and rented a car. (dh did the driving!). For architecture Florence and Rome would be your best bets. But Pompeii was fabulous and I definitely wouldn't skip that. I would skip Pisa except for his interest in architecture :) it's very touristy but we did it in < a day from Florence.
Maybe have your ds study and devise a game plan that interests him? As much as I love London (lived there for 2+ years, would go back in a heartbeat) and Paris, in your shoes I would focus on Italy and save those cities for another time. :)