got both sides of the shot issue here, some say use bigger shot, others say there isn't enough pellets, so use smaller shot.
7 1/2 or 8s are dove loads. My experience is that they do fine within their range. Going to the .410 means its harder to put enough pellets on the target, NOT that the pellets are any less effective. One or two #8s from a 12ga are exactly the same as one or two #8s from a .410 in my experience.
Solid hit with the center of the shot column always works, .410 or 12ga. The one real difference is that with a .410 there is sometimes something left.
Seriously, its the difficulty hitting moving targets that makes the .410 an "experts gun". If your boy is shooting clays and doing acceptably well, he should do about the same on live birds, once he adjusts to they way they behave.
You might consider finding some very light 12ga loads (trap loads?) and have him try them when he is ready (and he will be ready before you know it).
.410 shells in the larger shot sizes are meant for ground shooting of pests and small game more than they are for wingshooting larger birds. One can do it, but the difficulty often turns off beginners. Or turns them off to the .410, at any rate.
at 14, I got the use of a .410, and my first outing wasted an entire box of shells without a single hit, other than the one fencepost I shot (which, fortunately wasn't moving). The next time, I borrowed my Grandfather's 12ga double, and came back with some birds, and a split lip! The time after that, I came back with just some birds. Have used 12ga ever since. I have a couple of .410s (one bolt action, and a T/C Contender pistol), but don't use them for wingshooting. Fine pest guns though.