Stay away from the .410 for deer. Their ballistics are junk.

Most consider 1000 foot pounds of energy the bare minimum for deer. The best slugs I could find for a .410 produced only 760 foot pounds AT THE MUZZLE. Imagine what that energy drops to at 40 yards. Not to mention the fact that a .410 slug (1/5 oz) is barely over 100 grains. 100 grains is fine at 3000 fps, but not at 1200.

I'm a .410 nut, don't get me wrong. (I own7)I think it's the most under-rated caliber for small game. BUT, it has its limitations.

Get him a .45 caliber muzzleloader.