I think you have to have a certain mind set when using any subgauge. When I started duck hunting lead was legal and we started the season with 1 oz of 7 1/2 and switched to 1oz of 6's in our 12's later in season hunting public lands in WI. We did well. When I moved to MN in the 70's we had to switch to steel and again never had the problems that others had, we still used our skeet choked guns and the ducks kept falling we just let them in close enough to kill with steel or light lead.
I'm out on the pacific coast now and still hunt public lands and do just fine with 7/8-1oz of #4steel out of my little 20ga, 3/4 oz of ITX 6'd in my 2.5" 16 and 7/8 oz of the same out of my 2.5"12's.
The big thing is not competing with other hunters, pick places where the ducks want to be and let the marginal shots go. Small decoy spreads in back corners of the marsh will draw birds after they've been blasted at over the big spreads out on the open water. Hunt places that you can't get to with a 16 foot jon boat. Lose the boat blind, many times I will sit on a bare bank with a couple of decoys right in front of me and the ducks will drop right. Don't burn out a spot, I have a spot about a 100 yards down the beach from a very popular point with a nice natural blind on it, you can just watch the ducks come down the beach, swing out around the point and swing back into my decoys. I only hunt it on days with the correct tide and wind.
I'd rather go out and shoot at five ducks in a day and bag them than blast away all day. I read about guys laying in a few flats of shells for the season, I can't imagine having to have that many shells to hunt ducks and I eat duck steady through the season.
I can see where your son would be frustrated with the 20 if your shooting a 12 and dropping ducks out near the limit of the 12 and not letting them in close enough for the twenty, remember he's learning to judge distance by what he's seeing you do. I've gone on hunts with guys that had me outgunned and suffered through a day of never getting a flock in close enough to shoot.
My go to duck guns, 20ga CZ Bobwhite and 12ga 2.5" Husqvarna 51 IC & MOD