I guess there's some justification for a 3 1/2" gun for geese, but I wonder if passing on a few shots a season might make more sense even there.
$700 for a do all shotgun is actually a pretty hefty spend. You can get a Remington 870, 3" Express for under $400, or a 3 1/2" Supermag for about $600. Extra barrels run $175-$250 or so. The synthetic models come with a very nice adjusatable length of pull, at 15 and 5'11", my bet is he'll need the extra length by the time he's 18. I have that on my new 20Ga. Synthetic Camo, it is a very nice feature.
I recently picked up a 26" VR Express barrel for my 3" Express, and they now come chambered for 3 1/2" shells. It seems to pattern well with 2 3/4" and 3" stuff. Of the four 870's in the family, the oldest is about 25 years, and over 100 deer old. The limit here in NJ is 1-2/day from December through February. It still feeds and functions perfectly. My new 20Ga. is very nice, and I actually like it a lot better for turks as the weight is sweet at about 6lbs, and 1 1/4oz 3" loads are as effective as anything I've ever used out to 35yds.
Mod 1100's are nice, but they are heavier, more expensive, and more prone to malfunction over long ownership. They kick a lot less.