Skagan: The BBs your son is shooting measure .177 " while the BB steel shot is .18". the ' steel " shot BB is made of a much softer steel than your son's BB are. They are not interchangeable. He can ruin his rifle trying to shoot a BB made for shotguns from steel, or lead, for that matter. It will stop in the barrel, and you will ruin the barrel getting it out.
As for using them for waterfowl, if you actually price your son's BBs, they are one heckuvalot more expensive than even the expensive steel shot BBs will cost you. Those plastic shot cups, even the heavy gauge ones from Ballistics Products have been known to cut through, and if a steel BB touches the bore of your shotgun going out, it will scratch it. If you are shooting a gun with choke, it will scratch right through that, too.
I have been reading Mr. Nesbitt's work for many years, and I learned to ignore most if it long ago. If it works for him, fine. I would rather stick with the products made for the gun I am using. MY 2 cents.