You could get him an TSX or NYSE account and let him paper trade. Most major investment services and stock exchanges offer you an option to build a portfolio to monitor. You could build up a few industry or regional portfolios, a diversified one, and compare them against a broad spectrum index fund, like the one that tracks the S&P500.
If he does well with that, you could create a paper margin account and teach him simple puts and calls. I don't think it's anything a highly capable and motivated child like your son couldn't handle.
