IMO for waterfowl with a slight framed person, 20 ga is ideal. Of the gauges i would see the popularity as being 12, 20, 410, 28, and 16. The 16 is definitely least popular, with the recent resurgence of the 28 perhaps the 16 can also re appear. I know getting ammo for 20 is much easier and less expensive than either 28 or 16. Most ammo in those two will also be for upland and not steel.
Did not see a budget figure. So say someone picked up a 20 in a gun with adjustable spacers for LOP. Then what action. In new a simple 20 pump or SXS could be had for about $600, whereas, a high end semi would be $2000+. All of this is budget related, and depends on if this is a gun for 5 years or until he is old enough to get a lifetime full sized waterfowl gun.
My vote would go for something like a Weatherby automatic, Franchi affinity, Stoeger. In say a pump, see what is available in a Rem 870 compact.
My 2cents...round it up to 5 since the penny.