Wow, I never heard of anyone being bothered by the recoil of a .410. Part of the problem could be that the shotguns he shot were ill fitting, a shotgun that doesn't fit the shooter can enhance felt recoil. Many times people give kids guns that are too long or they are not holding the butt stock tight against the shoulder and the stock slams into the shoulder instead of pushing against it. I would check to see if the guns fit and how the boy is holding them. Single shot shotguns are notorious for not fitting. Before yousend money on another gun I would work on checking the fit and the way the boy hods the guns. Another consideration if the fit and hos shooting technique are not the problem is to go to a gas operated semi auto shot gun with a good recoil pad.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!