i brought up my 2 boys on .410's. they are much harder to learn to shoot with than 28, or 20. but once they got their leads down, they did pretty good. my younger one shot clay targets at camp for 2 summers before graduating to bobwhites. i even gave him a choice of guns, SxS, O/U, and he picked the 410 pardner.
i've taught a few kids with shotguns and my direct experience was that if the kid has a short attention span, and who doesn't or didn't, there is no instant gratification, so they get frustrated. if he is the pulling the trigger with his head up and the gun an inch or so off his shoulder, he will both always shoot high and miss, and get a nice little kick from the gun. if a kid was afraid of recoil, i had him hold a 2 x 4 like a gun and pull it hard into his shoulder, then get his stance right. i would give the 2 x 4 a little pop on the end to simulate recoil. that usually made them smile and say "is that all ?"
28 gauge is not the magic starting gauge. size the gun and the load to the kid, get his stance right, and teach him how to shoulder a shotgun before he ever fires a gun. and, don't start him out with grampa's 4 pound 12 gauge side by side guild gun that kicks like a mule with skeet loads !
a real configence builder is taking the kid out to a skeet field and him shoot at low 7's. this usually makes them grin from ear to ear !
sg