Whether this would work for your boy, but my friend's son was determined to muzzle load hunt this year so they got him a Tradition's Springfield Hawkins in .50 caliber. This young man is by no means a large young man, but he is shooting 70 grains and a patched roundball. The rifle seems small enough for his little frame to handle the rifle very well.

I think NEF made a youth model in the Sidekick type rifle also.