The last thing anyone wants to be told is that they don't want what they say they want. But he I am. My opinion is that a .410 is an expert's gun or a very specialised gun for shooting decoyed pigeons, rabbits back when and how this was done before myxomatosis, and squirrels. For all else I'd beg you to buy the boy a 28 bore. Truly, really, honestly. Because for the most part all .410" guns to teach a child come too tightly choked.
Some will say that a 21 gram 28 bore is little different to a 19 gram .410" but it isn't. It's a whole different theung and going forward to the demise of lead has a whole better future. Indeed I'd almost say buy him a 20 bore and that advice...the 20 bore option...then all of a sudden you've a whole spectrum of good, well made, reliable but inexpensive secondhand guns.
On the clay ground where I now mostly shoot once a week they start kids on a 20 bore. My father back in 1919 on his then twelfth birthday was started on a 12 bore and I have it still today although the stock that was cut down for him then lengthed as he grew with black ebonite I have now had replaced. That may be an extreme example though! But a .410" no.
But the point that I hope does not offend is as in my first paragraph. The .410" is excellent in a particular niche. I have one a Webley & Scott bolt action in the Deluxe version. But starting a child to learn on one will produce misses and that will give disappointment. Leave the .410" to its forte or to the experts. Look for a 28 bore at the minimum but better a 20 bore (with unfashionable twenty-six inch or twenty-eight inch barrels) instead.