30-30, I would have to say. Or .243. Either one would be fine.

Hodgedon put out some great reduced loads for youth for a whole slew of calibers with H4895. Only problem will be most 30-06's and so are not made for youths and stock won't fit them that well. If your a reloader, and would want to get him a gun that would fit him for a long time, maybe a model 7 .308 and make some nice reduced loads.

Just a thought.