Mauser,
Heck, I killed my first squirrel at age 7 with the ol' 20 guage side by side. I know plenty of kids that start hunting and have success that young. You can start him on the play guns now, then on to BB guns and then the real thing. Just be sure to emphasize woodsmanship and safety above all else. Thats what my dad did with my brothers and me and what I plan on doing with my daugther (she's 3 also). With sound teaching and proven trust, my dad was able to let me start hunting by myself (with him still in the vacinity) by the time I was 11.
The ethics and morals that come from the outdoors experience and country life in general are priceless.
Smoky