If you got him a H&R 20ga single shot slug gun, which has a fully rifled barrel, you could have him shoot the Remington Buckhammer Managed-Recoil slugs to start, and as he grows into the gun he could move up to full power shells in the future.

I'd be a little reluctant to use a .410 bore for deer, and definitely not with buckshot. Slugs maybe at under 50 yards. A 20ga will serve him better as he grows. I got my first 20ga when I was 8, and I handled everything up to heavy field pheasant loads just fine.

Mike