The more basic, the more he'll learn. How about a Spotmatic, or even a non-metered SLR and a separate meter? I started when 11 years old with a non-metered camera (and no meter!) and soon became quite good at gauging exposures - always a useful skill to have.
I would say the best bet is to give him the A-1. It's good enough. If you were to buy him another it has to be better than the A-1 and not less as it would short change his talent. But for another camera that is more capable than the A-1? I think the requirement should come from him after using the A-1 for a while.
Make his camera a fully manual type. A camera with the capabilities of a Pentax K1000 would be a good choice. Get a seperate light meter. I beliive by starting out fully manual camera without any auto exposure or auto focus is going to help your son to learn more quickly the principles of photography that he needs to know to have adequate control and to be able to learn from his mistakes.
Add another body and a couple of lenses to the Canon stable. Youve already got some, so having a "spare" body and greater lens selection, is economical, and it makes good sense to keep another body. because the one you have will go out before the lenses. When he won't get out of your MF stuff, then you can worry about something else. I shot with the same AE-1 from the age of 13 to the age of 30.
I really disagree about the fully manual/separate meter route. Intellectually, it's attractive, and it's the way many of us learned. That doesn't mean it's the best route for a 10-year-old in 2006.
I don't know the boy in question, obviously (though I like him already!) but I DO know, both from teaching in schools and from young friends 10-15 who have taken an interest in photography, that if it gets too much like hard work, there are far too many other distractions available and they will be taken.